Dissertations / Theses on the topic 'Astronomia X'

El objetivo principal de esta tesis ha sido el estudio del comportamiento transitorio durante estallidos de distinto tipo, de una selección de pulsares acretores en rayos X, localizados en el plano galáctico.Con ello se pretende haber avanzado hacia una explicación más clara de la naturaleza de estas objetos binarios de alta masa (con estrella de neutrones como objeto compacto), así como de los mecanismos físicos que operan en este escenario. Para todo esto se han analizado datos de dos misiones espaciales:INTEGRA Y RXTE.El segundo objetivo ha sido el estudio de la zona del brazo galactico de Scutum. Al ser esta una región de fuerte absorción se cree de la existencia de sistemas del tipo anteriormente mencionado este todavía escondidos, y los cuales solo pueden ser descubiertos a altas energías. En esta tesis se han podido presentar resultados esperanzadores, con la detección de varios objetos posibles nuevos candidatos a pulsares acretores de rayos X.

The high energy astrophysics, specifically the gamma-ray astrophysics, studies the processes that cannot be caused by hot matter but by other mechanisms colled non thermaland processes, which involve matter with energies above ~ 1 MeV. There are currently a number of instruments able to detect this radiation, such as AGILE and Fermi satellites or Cherenkov telescopes like MAGIC at the Earth's surface. In this thesis we studied two main types of systems that, as it has been observed or theoretically predicted, can produce gamma radiation : young stellar objects and high-mass X-ray binaries. Young stellar objects are found in regions of star formation, which are the cradles where new stars are being formed. The star-forming regions are populated with protostars and young stars, among other objects. Protostars still accrete matter from the parent cloud through an accretion disk, while expelling material through a jet formed by magnetic interaction. In these jets the particles are ejected at high speeds out of the system and in some cases reach relativistic velocities, as evidenced by the detection of non-thermal radio emission in some of these objects. In this thesis we are interested to find more evidence of young stellar objects that present non-thermal emission, either in the range of X-rays or gamma rays. To find new candidates have used the first catalog of the Fermi satellite and the catalogs of young systems in our Galaxy. Besides the search through catalogs, we did a study using archival X-ray XMM-Newton data on the young stellar object IRAS 16547-4247, which is known to display non-thermal radio emission. This object is a protostar still accretes material through an accretion disk and also ejects jets of material through particle. We discovered its X-ray counterpart and studied the interaction of its jets through a theoretical model. Finally, we studied a region of star formation that has been found in spatial coincidence with a second Fermi source catalog, known as Monoceros R2 . We analyzed the Fermi data from this source and our results allow us to say that the detected gamma-ray emission is consistent with that expected by the collective effects of a population of protostars. Among the other large block of this thesis there are high-mass X-ray binares. The most relevant of which we have studied is MWC 656. This system consists of a Be star and a black hole, a combination that has never been detected before. We have observed this source with XMM-Newton in X-rays and with the MAGIC Telescopes in very high energy gamma rays. Our X-ray observation has led to the discovery of the X-ray counterpart of this binary system and has allowed us to classify it as a high-mass X-ray binary, the first composed of a Be star and a black hole. Other systems we studied with MAGIC are HESS J0632 +057 and SS 433. HESS J0632 +057 is a binary system consisting of a Be star and a compact object of unknown nature and was identified by us as a gamma-ray emitter. We have also observed SS 433, the first microquasar ever discovered. We observed this source during 2010 May and June, but it has not been detected. We have calculated upper limits to the emission of very high energy gamma-rays to put constraints on the physical parameters of the system.<br>L'astrofísica d'altes energies i en concret l'astrofísica de raigs gamma, estudia els processos d'emissió que no poden ser causats per matèria calenta, sinó per altres mecanismes que anomenem no tèrmics i que comporten que la matèria que emet aquesta radiació tingui energies per sobre d'1~MeV. Actualment disposem d'un bon nombre d'instruments capaços de detectar aquesta emissió, com ara els satèl•lits Fermi i AGILE o els telescopis Cherenkov com MAGIC, a la superfície terrestre. En aquesta tesi hem estudiat principalment dos tipus de sistemes que, tal com s'ha observat o predit teòricament, poden produir radiació gamma: els objectes estel•lars joves i els sistemes binaris de raigs X d’alta massa. Els objectes estel•lars joves els trobem a les regions de formació estel•lar, que són els bressols on noves estrelles s'estan formant. Els pobladors de les regions de formació estel•lar són les protoestrelles i les estrelles joves, entre altres objectes celests. Les protoestrelles encara acretem matèria del núvol progenitor a través d'un disc d'acreció, i al mateix temps expulsen material per mitjà d'uns dolls formats per interacció magnètica. En aquests dolls les partícules són expulsades a grans velocitats cap a fora del sistema i en alguns casos assoleixen velocitats relativistes tal com evidencia la detecció d'emissió ràdio no tèrmica en alguns d'aquests objectes. En aquesta tesi ens hem interessat per trobar evidència de més objectes estel•lars joves que presentin emissió no tèrmica, ja sigui en el rang dels raigs X o dels raigs gamma. Per a trobar nous candidats hem aprofitat el primer catàleg del satèl•lit Fermi i l'hem creuat amb catàlegs d'objectes joves de la Galàxia. A més a més de la cerca per mitjà de catàlegs, hem fet un estudi en raigs X a partir de dades d'arxiu disponibles sobre un objecte estel•lar jove del que ja es coneix emissió no tèrmica en ràdio: IRAS 16547-4247. Aquest objecte és una protoestrella que encara acreta material per mitjà d'un disc d'acreció i que alhora expulsa material a través de dolls de partícules. Hem descobert la contrapartida en raigs X d'aquest sistema i l'hem estudiat per mitjà d'un model teòric. Finalment, hem estudiat una regió de formació estel•lar que s'ha trobat en coincidència espacial amb una font del segon catàleg de Fermi, coneguda com Monoceros R2. Hem analitzat les dades de Fermi d'aquesta font i els nostres resultats ens permeten dir que l'emissió gamma detectada és compatible amb el que s'esperaria que produissin un conjunt de protoestrelles. Dins de l'altre gran bloc d'aquesta tesi trobem les estrelles binàries de raigs X d'alta massa. El cas més rellevant dels que hem estudiat és el de MWC 656. Aquest sistema està format per una estrella Be i un forat negre, una combinació que mai s'havia detectat. Nosaltres hem observat aquesta font amb el telescopi de raigs X XMM-Newton i amb els Telescopis MAGIC, en raigs gamma de molt alta energia. La nostra observació de raigs X ha suposat la descoberta de la contrapartida de raigs X d'aquest sistema binari i ens ha permès classificar-la com a binària de raigs X d'alta massa, la primera composada per una estrella Be i un forat negre. Altres sistemes que hem estudiat amb MAGIC són HESS J0632+057 i SS 433. HESS J0632+057 és un sistema binari format per una estrella Be i un objecte compacte de natura desconeguda i va ser detectat per nosaltres com a emissor de raigs gamma. També hem observat SS 433, el primer microquàsar que es va descobrir. Hem observat aquesta font durant els mesos de maig i juny de 2010 però no s'ha detectat. Així, hem calculat límits superiors a l'emissió gamma de molt alta energia que serveixen per a posar restriccions en paràmetres físics.

In this Thesis we have presented our work on the analysis of galaxy clusters through their X-ray emission and the gravitational lensing effect that they induce. Our research work was mainly finalised to verify and possibly explain the observed mismatch between the galaxy cluster mass distributions estimated through two of the most promising techniques, i.e. the X-ray and the gravitational lensing analyses. Moreover, it is an established evidence that combined, multi-wavelength analyses are extremely effective in addressing and explaining the open issues in astronomy: however, in order to follow this approach, it is crucial to test the reliability and the limitations of the individual analysis techniques. In this Thesis we also assessed the impact of some factors that could affect both the X-ray and the strong lensing analyses.

In this Thesis we have presented our work on the analysis of galaxy clusters through their X-ray emission and the gravitational lensing effect that they induce. Our research work was mainly finalised to verify and possibly explain the observed mismatch between the galaxy cluster mass distributions estimated through two of the most promising techniques, i.e. the X-ray and the gravitational lensing analyses. Moreover, it is an established evidence that combined, multi-wavelength analyses are extremely effective in addressing and explaining the open issues in astronomy: however, in order to follow this approach, it is crucial to test the reliability and the limitations of the individual analysis techniques. In this Thesis we also assessed the impact of some factors that could affect both the X-ray and the strong lensing analyses.

I quasars Radio Loud (RL) fanno parte di una piccola percentuale (~10%) di Nuclei Galattici Attivi (AGN) e sono caratterizzati da una forte emissione nella banda radio. Lo scopo del nostro lavoro e’ lo studio dell’emissione nella banda dell’X e dell’hard-X di una sotto-classe di questi oggetti: i Blazars. Gli oggetti di tipo Blazar sono AGN aventi un getto di particelle relativistiche (per lo piu’ elettroni e protoni) orientato a piccoli angoli rispetto alla linea di vista. Questa particolare classe di AGN include oggetti di tipo BL Lacerate (BL Lac) e Flat Spectrum Radio Quasar (FSRQ), che si differenziano per gli spettri ottici e per l’intensita’ della loro emissione: una sorgente BL Lac non presenta righe di emissione ottiche (EW < 5 Amstrong) e viene usualmente classificata come un blazar di bassa potenza (LBol~1046 – 1047 erg s-1), mentre una sorgente FSRQ mostra righe di emissione con significative larghezze equivalenti ed e’ classificata come un blazar di alta potenza (LBol~1048 erg s-1). In generale, lo spettro di un AGN in banda X e’ ben modellato da una legge di potenza con un flusso specifico (per unita’ di intervallo di energia) della forma N(E) ~ E-Gamma, dove E e’ l’energia in keV, N(E) e’ il numero di fotoni in unita’ di s-1 cm-2 keV-1 e Gamma e’ chiamato indice fotonico. Nonostante questa visione chiara e generale, lo spettro X di un Blazar puo’ mostrare alcune deviazioni dalla semplice legge di potenza sopra menzionata. Queste particolari caratteristiche spettrali si manifestano nel soft X con una curvatura dello spettro: un appiattimento (o flattening) e un irripidimento (o steepening) dello spettro. Inoltre, la presenza di componenti di riflessione Compton del continuo intrinseco – piu’ comunemente note come un picco (o Compton reflection hump) a 20-30 keV ed una riga di fluorescenza del ferro K-alpha – in oggetti RL e’ ancora una questione molto dibattuta. Ad oggi non e’ stata ancora trovata una chiara interpretazione fisica di queste caratteristiche spettrali. In questo lavoro riportiamo i risultati di uno studio a larga-banda (0.2-100 keV) dell’emissione nell’X di un campione selezionato di 11 Blazars osservati con INTEGRAL: 1ES 0033+595, 3C 273, 3C 279, 4C 04.42, BL Lac, IGR J22517+2218, PKS 0537-286, PKS 1830-211, PKS 2149-306, QSO B0836-710 e Swift J1656.3-3302. L’osservatorio spaziale INTEGRAL, grazie al largo campo di vista e alla alta risoluzione spettrale dei suoi strumenti, e’ ideale per studiare l’emissione di sorgenti X alle alte energie (oltre i 10 keV). Lo scopo di questo lavoro di Dottorato e’ l’analisi di un campione di Blazars selezionato nell’hard X ed avente dati disponibili provenienti dai satelliti INTEGRAL, Swift e XMM-Newton. Dopo aver eseguito la procedura standard per la riduzione dati nel soft X ci siamo occupati dell’analisi temporale e spettrale dell’intero campione di sorgenti. Riassumiamo di seguito i risultati scientifici ottenuti: • gli spettri a larga banda di tutte le sorgenti – eccetto due, le FSRQs 4C 04.42 e 3C 273 – sono ben riprodotti con un modello a legge di potenza assorbita da una quantita’ di gas in eccesso a quella galattica (NH~1020-23 cm-2). Questo risultato e’ una chiara evidenza del fatto che, utilizzando dati INTEGRAL alle alte energie, le sorgenti pesantemente assorbite possono essere piu’ facilmente rivelate. Inoltre, l’assorbimento sembra provenire da un materiale freddo intrinseco, confermando ed estendendo precedenti risultati presentati in letteratura. Il presente lavoro fornisce un’ulteriore conferma dell’esistenza di una correlazione tra l’NH ed il redshift; • per quanto concerne il continuo di emissione, l’analisi a larga banda del nostro campione di Blazars ha evidenziato la presenza di spettri piu’ hard (con indici fotonici dell’ordine di 1.4) rispetto a quelli di un campione piu’ ampio di oggetti RL analizzato da Page e collaboratori nel 2005 con dati di XMM-Newton. Tale differenza potrebbe risiedere nel fatto che effettivamente INTEGRAL seleziona sorgenti con valori dell’indice fotonico piu’ piatti, come mostrato anche da precedenti risultati; • nella nostra analisi abbiamo trovato due casi (4C 04.42 e 3C 273) di eccesso di conteggi nel soft X rispetto al modello di semplice legge di potenza (in altre parole uno steepening dello spettro), indicando la possibile presenza di un’emissione nel soft X; • non abbiamo trovato evidenza di componenti di riflessione (ne’ di Compton Reflection hump ne’ di una riga di emissione del ferro). La struttura della tesi e’ la seguente: Nei capitoli 1 e 2 riassumiamo le proprieta’ osservative comuni a tutte le sotto-classi di AGN enfatizzando in particolare la classe dei Blazars, diamo una descrizione generale della loro distribuzione spettrale in energia e discutiamo i loro meccanismi di emissione nella banda delle alte energie e le questioni ancora aperte sugli spettri degli oggetti RL. Nel capitolo 3 introduciamo la missione INTEGRAL e ne confrontiamo i rivelatori X a bordo con quelli di altri strumenti quali, BAT e XRT a bordo del satellite Swift e MOS1, MOS2, PN a bordo di XMM-Newton. Nel capitolo 4 riportiamo il metodo di riduzioni dei dati da noi adottato e l’analisi temporale e spettrale del campione selezionato di Blazars. Infine, nel capitolo 5 discutiamo i risultati ottenuti e diamo le nostre conclusioni.<br>Radio Loud (RL) quasars represent a small percentage (~10%) of all Active Galactic Nuclei (AGN) with a strong radio emission. We aimed to study the X-ray and hard X-ray emission from a subclass of these objects: the Blazars. The Blazar objects are AGNs with a jet of plasma (largely formed by electrons and protons) emitting at low angles with respect the line-ofsight. This class of AGN include BL Lacertae objects (BL Lac) and Flat- Spectrum Radio Quasars (FSRQ). The intrinsic differences between two types of sources are been found in their optical spectra and in the intensity of their emission: a BL Lac source does not show any optical emission lines (EW < 5°A) and this class of objects is essentially a low-power blazar (LBol~1046 − 1047 erg s−1), whereas a FSRQ source shows significant emission line equivalent widths and corresponding to a high-power blazar (LBol~1048 erg s−1). In general, the spectrum of an AGN in the X-rays band is usually well described by a power-law with a specific flux (i.e. per unit energy interval) of the form N(E) / E−Gamma, where E is the energy, N(E) is the number of photons in units of s−1 cm−2 keV−1 and Gamma is called the photon index. Despite this general view, X-ray spectra of the Blazars show some deviations from the simple power-law. These spectral signatures appear in the soft X-ray band with a curvature (a flattening or a steepening of the spectrum). In addition, the presence of Compton reflection components - most notably the Fe K-alpha emission line and the Compton reflection ”hump” peaked at about 20−30 keV - in RL objects is still debated. To date a clear physical interpretation of these features has not yet been found. We report in this work the results of a broad-band (0.2 − 100 keV) study of X- ray emission from a selected sample of 11 Blazars detected by INTEGRAL: 1ES 0033+595, 3C 273, 3C 279, 4C 04.42, BL Lac, IGR J22517+2218, PKS 0537-286, PKS 1830-211, PKS 2149-306, QSO B0836- 710 and Swift J1656.3-3302. The INTEGRAL observatory, because of the large field of view and high spectral resolution of its instruments, is ideal to study the high energy emission (above 10 keV). Present PhD work was aimed to the investigation of this hard X-rays selected sample, by taking advantage of the availability of the INTEGRAL, Swift and XMM-Newton data. After data reduction procedure, we performed a detailed temporal and spectral analysis for all sources in the sample. The scientific results can be summarized as follows: • the broad-band spectra of all selected sources - but two, the FSRQs 4C 04.42 and 3C 273 - are well reproduced with a power-law model absorbed by an amount of gas in excess to the Galactic one (NH ~ 1020 − 1023 cm−2). This result provides plain evidence to the fact that by using the INTEGRAL high-energies data the highly absorbed sources can be more easily detected. Moreover, the absorption seems to be a signature of a cold intrinsic absorber, confirming and extending previous results quoted in the literature. Present work provides a further confirmation of the existence of a NH-redshift trend; • with regard to continuum emission we found that the broad-band analysis of our sample of Blazars revealed a harder spectrum with a photonindex of the order of 1.4, with respect to the mean value of the spectral index obtained by Page et al. (2005) with XMM data of a statistically sizeable sample of RL objects. Such a difference could be due to the hard X-ray selection of our INTEGRAL sample that is clearly biased towards flatter values of the photon index as shown by previous results; • in our analysis we have find two case (4C 04.42 and 3C 273) of excess of soft X-ray counts with respect to the simple power-law model (in other words a steepening of the spectrum), indicating the possible presence of a soft X-ray emission; • we have not found any evidence of reflection components (Reflection ”hump” and iron emission line). The thesis structure is as follows: In Chapters 1 and 2 we summarize the observational properties common to all AGN subclasses emphasizing in particular the Blazar class, we give a general description of their spectral energy distribution and then we discuss their emission mechanisms in the high energy band and the open questions related to the spectra of the RL objects. In Chapter 3 we deal with the INTEGRAL mission overview and a comparison with some X-ray detectors such as, Swift/BAT, Swift/XRT and XMM-Newton. In Chapter 4 we report on the data reduction method and analysis of the Blazar selected sample. Finally, in Chapter 5 we discuss the results and give our conclusions.

In questo elaborato si vuole descrivere il funzionamento generale di telescopi, in banda ottica, radio e X, con un approfondimento sui principi di funzionamento e sulle relative differenze. In generale si può definire un telescopio come uno strumento capace di raccogliere la radiazione elettromagnetica per concentrarla grazie a lenti, specchi o antenne paraboliche, in un’area ristretta, un punto focale in cui si trova un rivelatore, che può essere l’occhio umano, un CCD, un illuminatore ecc. Questi, grazie anche a software dedicati, elaborano la radiazione raccolta fornendo un’immagine della sorgente celeste.

Per telescopio intendiamo qualsiasi strumento finalizzato alla misura della radiazione proveniente dallo spazio. Tipicamente questo nome viene riservato agli strumenti ottici; tuttavia è utile utilizzare un singolo nome per caratterizzare tutta la classe di strumenti per le osservazioni astronomiche. Un telescopio è uno strumento capace di raccogliere radiazione da una grande superficie, concentrandola in un punto. La luce viene in genere raccolta da uno specchio o antenna, quindi elaborata da vari strumenti, come per esempio un filtro o uno spettrografo, e infine indirizzata ad un rivelatore, che può essere l'occhio umano, una lastra fotografica, un CCD, un rivelatore radio, una camera a scintille etc.

The hard X-ray band (10 - 100 keV) has been only observed so far by collimated and coded aperture mask instruments, with a sensitivity and an angular resolution lower than two orders of magnitude as respects the current X-ray focusing telescopes operating below 10 - 15 keV. The technological advance in X-ray mirrors and detection systems is now able to extend the X-ray focusing technique to the hard X-ray domain, filling the gap in terms of observational performances and providing a totally new deep view on some of the most energetic phenomena of the Universe. In order to reach a sensitivity of 1 muCrab in the 10 - 40 keV energy range, a great care in the background minimization is required, a common issue for all the hard X-ray focusing telescopes. In the present PhD thesis, a comprehensive analysis of the space radiation environment, the payload design and the resulting prompt X-ray background level is presented, with the aim of driving the feasibility study of the shielding system and assessing the scientific requirements of the future hard X-ray missions. A Geant4 based multi-mission background simulator, BoGEMMS, is developed to be applied to any high energy mission for which the shielding and instruments performances are required. It allows to interactively create a virtual model of the telescope and expose it to the space radiation environment, tracking the particles along their path and filtering the simulated background counts as a real observation in space. Its flexibility is exploited to evaluate the background spectra of the Simbol-X and NHXM mission, as well as the soft proton scattering by the X-ray optics and the selection of the best shielding configuration. Altough the Simbol-X and NHXM missions are the case studies of the background analysis, the obtained results can be generalized to any future hard X-ray telescope. For this reason, a simplified, ideal payload model is also used to select the major sources of background in LEO. All the results are original contributions to the assessment studies of the cited missions, as part of the background groups activities.

The hard X-ray band (10 - 100 keV) has been only observed so far by collimated and coded aperture mask instruments, with a sensitivity and an angular resolution lower than two orders of magnitude as respects the current X-ray focusing telescopes operating below 10 - 15 keV. The technological advance in X-ray mirrors and detection systems is now able to extend the X-ray focusing technique to the hard X-ray domain, filling the gap in terms of observational performances and providing a totally new deep view on some of the most energetic phenomena of the Universe. In order to reach a sensitivity of 1 muCrab in the 10 - 40 keV energy range, a great care in the background minimization is required, a common issue for all the hard X-ray focusing telescopes. In the present PhD thesis, a comprehensive analysis of the space radiation environment, the payload design and the resulting prompt X-ray background level is presented, with the aim of driving the feasibility study of the shielding system and assessing the scientific requirements of the future hard X-ray missions. A Geant4 based multi-mission background simulator, BoGEMMS, is developed to be applied to any high energy mission for which the shielding and instruments performances are required. It allows to interactively create a virtual model of the telescope and expose it to the space radiation environment, tracking the particles along their path and filtering the simulated background counts as a real observation in space. Its flexibility is exploited to evaluate the background spectra of the Simbol-X and NHXM mission, as well as the soft proton scattering by the X-ray optics and the selection of the best shielding configuration. Altough the Simbol-X and NHXM missions are the case studies of the background analysis, the obtained results can be generalized to any future hard X-ray telescope. For this reason, a simplified, ideal payload model is also used to select the major sources of background in LEO. All the results are original contributions to the assessment studies of the cited missions, as part of the background groups activities.

This Thesis focuses on the X-ray study of the inner regions of Active Galactic Nuclei, in particular on the formation of high velocity winds by the accretion disk itself. Constraining AGN winds physical parameters is of paramount importance both for understanding the physics of the accretion/ejection flow onto supermassive black holes, and for quantifying the amount of feedback between the SMBH and its environment across the cosmic time. The sources selected for the present study are BAL, mini-BAL, and NAL QSOs, known to host high-velocity winds associated to the AGN nuclear regions. Observationally, a three-fold strategy has been adopted: - substantial samples of distant sources have been analyzed through spectral, photometric, and statistical techniques, to gain insights into their mean properties as a population; - a moderately sized sample of bright sources has been studied through detailed X-ray spectral analysis, to give a first flavor of the general spectral properties of these sources, also from a temporally resolved point of view; - the best nearby candidate has been thoroughly studied using the most sophisticated spectral analysis techniques applied to a large dataset with a high S/N ratio, to understand the details of the physics of its accretion/ejection flow. There are three main channels through which this Thesis has been developed: - [Archival Studies]: the XMM-Newton public archival data has been extensively used to analyze both a large sample of distant BAL QSOs, and several individual bright sources, either BAL, mini-BAL, or NAL QSOs. - [New Observational Campaign]: I proposed and was awarded with new X-ray pointings of the mini-BAL QSOs PG 1126-041 and PG 1351+640 during the XMM-Newton AO-7 and AO-8. These produced the biggest X-ray observational campaign ever made on a mini-BAL QSO (PG 1126-041), including the longest exposure so far. Thanks to the exceptional dataset, a whealth of informations have been obtained on both the intrinsic continuum and on the complex reprocessing media that happen to be in the inner regions of this AGN. Furthermore, the temporally resolved X-ray spectral analysis field has been finally opened for mini-BAL QSOs. - [Theoretical Studies]: some issues about the connection between theories and observations of AGN accretion disk winds have been investigated, through theoretical arguments and synthetic absorption line profiles studies.

This Thesis focuses on the X-ray study of the inner regions of Active Galactic Nuclei, in particular on the formation of high velocity winds by the accretion disk itself. Constraining AGN winds physical parameters is of paramount importance both for understanding the physics of the accretion/ejection flow onto supermassive black holes, and for quantifying the amount of feedback between the SMBH and its environment across the cosmic time. The sources selected for the present study are BAL, mini-BAL, and NAL QSOs, known to host high-velocity winds associated to the AGN nuclear regions. Observationally, a three-fold strategy has been adopted: - substantial samples of distant sources have been analyzed through spectral, photometric, and statistical techniques, to gain insights into their mean properties as a population; - a moderately sized sample of bright sources has been studied through detailed X-ray spectral analysis, to give a first flavor of the general spectral properties of these sources, also from a temporally resolved point of view; - the best nearby candidate has been thoroughly studied using the most sophisticated spectral analysis techniques applied to a large dataset with a high S/N ratio, to understand the details of the physics of its accretion/ejection flow. There are three main channels through which this Thesis has been developed: - [Archival Studies]: the XMM-Newton public archival data has been extensively used to analyze both a large sample of distant BAL QSOs, and several individual bright sources, either BAL, mini-BAL, or NAL QSOs. - [New Observational Campaign]: I proposed and was awarded with new X-ray pointings of the mini-BAL QSOs PG 1126-041 and PG 1351+640 during the XMM-Newton AO-7 and AO-8. These produced the biggest X-ray observational campaign ever made on a mini-BAL QSO (PG 1126-041), including the longest exposure so far. Thanks to the exceptional dataset, a whealth of informations have been obtained on both the intrinsic continuum and on the complex reprocessing media that happen to be in the inner regions of this AGN. Furthermore, the temporally resolved X-ray spectral analysis field has been finally opened for mini-BAL QSOs. - [Theoretical Studies]: some issues about the connection between theories and observations of AGN accretion disk winds have been investigated, through theoretical arguments and synthetic absorption line profiles studies.

This Thesis focuses on NuSTAR hard X-ray surveys of Active Galactic Nuclei (AGN). One of the major goals of the NuSTAR mission is to study with unprecedented detail the obscured and heavily obscured (Compton-thick, CT) populations of AGN, significantly contributing to the diffuse cosmic X-ray background (CXB),and major actors within the evolutionary cycle of galaxies. In the first part, a survey of a small sample (∼ a dozen) of local (z < 0.03) heavily obscured AGN, selected by their water megamaser emission at ∼22.23 GHz, is presented. Thanks to the NuSTAR hard X-ray spectral coverage (extending between ∼3−80 keV), a robust estimate of the CT fraction is obtained. Building up on this result and combining X-ray and radio measurements, a toy model of the dusty megamaser disk within the classical dusty torus is proposed. A spin-off of the megamaser project is related to a well-known local obscured AGN (Mrk1210) and devoted to studying its long-term (∼17 yrs) X-ray variability with NuSTAR, from which some constraints on the AGN environment can be drawn. Finally, the search and study of CT AGN is pushed to higher redshifts, fully exploiting the NuSTAR X-ray capabilities in terms of sensitivity. The hunt for faint and distant heavily obscured AGN is performed exploiting the combined NuSTAR, XMM-Newton and Chandra coverages of the UKIDSS-UDS field. The deep NuSTAR survey strategy is presented, along with the modeling of the NuSTAR background in order to optimize the detection of faint sources. A broadband X-ray spectroscopic analysis of all the detected sources is performed, and combined with the standard hardness ratio (HR) diagnostic, in order to select all the possible CT candidates. Such CT candidates are then analyzed again with appropriate X-ray spectral models specifically developed to deal with CT absorbers.

Lo scopo di questo elaborato è di descrivere il funzionamento dei telescopi che lavorano nelle bande ottico, radio e X, sottolineandone i principi di funzionamento e le loro differenze. Nella stesura dell'elaborato ho cercato di seguire la linea storica degli eventi, partendo dalla descrizione dei telescopi ottici -i primi che sono stati costruiti- per poi passare ai radiotelescopi e ai più "giovani" telescopi X. Ho dedicato un ultimo capitolo all'approfondimento dell'evento GW170817 che ritengo un esempio utile a mostrare le diverse funzionalità di questi telescopi e allo stesso tempo l'importanza dell'uso sincronizzato di osservazioni a diverse bande per un'analisi degli eventi più dettagliata.

Le sorgenti ultraluminose in banda X (ULX) sono binarie X extragalattiche con luminosità maggiore del limite di Eddington per un buco nero (BH) di 10 Msun (L>10^39 erg/s). Si pensa siano alimentate nella maggior parte dei casi da un accrescimento super-Eddington su BH stellari o stelle di neutroni (NS). Solo in pochi casi siamo a conoscenza della natura dell'oggetto compatto, identificata grazie alla rilevazione di pulsazioni, che possono essere emesse solo da una NS. La frazione relativa di BH e NS nelle ULX e i dettagli dell'accrescimento super-Eddington sono ancora sconosciuti. In questa tesi mi sono concentrata sull'analisi della variabilità nelle ULX, analizzando dati in banda X, che è legata ai processi di accrescimento, per cui è utile per ottenere informazioni sulla fisica dell'accrescimento super-Eddington. Ho analizzato la variabilità a lungo termine (tempi scala di giorni) di 24 ULX in galassie a spirale, che abbiamo monitorato con il satellite Swift. La variabilità è significativa nel 71% delle sorgenti e in tutte le ULX variabili ha un'ampiezza >30%. Ho stimato l'ampiezza della variazione con la fractional variability e questo è il primo studio di ULX nel quale questo stimatore è usato sistematicamente su questi tempi scala. Nel 53% delle ULX variabili, la variabilità è guidata dalla banda hard. Nello scenario super-Eddington l'accrescimento avviene in un disco modificato, caratterizzato da avvezione e venti e geometricamente spesso all'interno del raggio di sfericizzazione, dove si raggiunge il limite di Eddington. Lo spettro può essere modellato con 2 componenti termiche: la più fredda è associata alla fotosfera del vento o alla parte esterna del disco e la più calda alla parte interna del disco. La variabilità della banda hard è associata alla componente spettrale più calda. Ho interpretato la variabilità osservata nella banda di energia totale come conseguenza di un tasso di massa di accrescimento variabile. La variabilità addizionale in banda hard può derivare dall'oscuramento delle regioni interne del disco dal vento soft che può essere lungo la nostra linea di vista o fuori da essa durante le diverse osservazioni. La maggior parte delle sorgenti ha uno spettro consistente con un regime di accrescimento ultraluminoso. In NGC925 ULX-3 abbiamo trovato una periodicità nel flusso di circa 4 mesi (Salvaggio et al., 2022), simile ai periodi trovati in altre ULX e probabilmente legato a una modulazione super-orbitale. Ho trovato alcune candidate NS nel campione analizzato e un'attività di flaring in un'altra ULX del campione. Ho osservato transizioni spettrali in 4 sorgenti e stimato un tempo scala di qualche mese per le transizioni. Ho anche studiato la variabilità su tempi scala di anni in un campione di ULX in una galassia ad anello, la Cartwheel: 35% variano significativamente e la percentuale potrebbe essere maggiore data la bassa statistica dei dati analizzati. Nel 40% delle sorgenti ho osservato un comportamento transiente. Ho anche studiato la funzione di luminosità in X (XLF) per vedere se varia tra diverse epoche. Ho trovato che la forma della XLF è costante, nonostante la variabilità nel flusso delle ULX.<br>Ultraluminous X-ray sources (ULX) are extragalactic X-ray binaries emitting luminosities in excess of the Eddington limit for a 10 Msun black hole (BH) (L>10^39 erg/s). They are thought to be powered in most cases by super-Eddington accretion onto stellar mass BHs or neutron stars (NS). Just in few cases the nature of the compact object has been identified, through the detection of pulsations, which can be only emitted by a NS. The relative fraction of BHs and NSs in ULX and the details of super-Eddington accretion physiscs are still unknown. In this thesis I focused on the analysis of the variability in ULX, by analysing X-ray data, which is linked to the accretion processes and thus can help to obtain information on super-Eddington accretion physics. I analysed the long-term variability, on days timescales, in a sample of 24 ULX in spiral galaxies, which we monitored with the Swift satellite. The variability is significant in 71% of the sources and in all the variable sources the variability amplitude is larger than 30%. I used the fractional variability to estimate the variability amplitude and this is the first study of ULX in which it is used systematically on such timescales. In 53% of the variable sources the variability is driven by the hard energy band. In a super-Eddington accretion scenario, the accretion happens in a modified disk structure. The disk is characterized by advection and outflows and becomes geometrically thick inside the spherization radius, where the Eddington limit is reached. The spectrum can be modelled with 2 thermal components: the colder one is associated to the photosphere of the wind or the outer disk and the hotter to the inner disk. The variability of the hard band is associated to the hotter spectral component. I interpret the variability in the total band as a consequence of a variable mass accretion rate. The additional variability in the hard band may be caused by obscuration of it by the soft wind component that may be along or out of our line of sight among the observations. Most of the sources have a spectrum consistent with an ultraluminous accretion regime. In NGC925 ULX-3 we also found a periodicity in the flux of about 4 months (Salvaggio et al., 2022), similar to periods found in other ULX and probably linked to a super-orbital periodicity. I found some candidate NS in the sample and the presence of a flaring activity in another ULX. I found spectral state transitions in 4 ULXs and estimated a timescale of months for the transition. I’ve also studied the variability on year timescales in a sample of ULX in a ring galaxy, the Cartwheel: 35% of them vary significantly and this percentage may be larger considering the low statistics of the data. In 40% of the ULX I observed a transient behaviour. I also studied the X-ray luminosity function (XLF) to see if it is variable among different epochs. The XLF is consistent with a constant shape, despite the flux variability of the ULX.

La polarimetria è l'ultima branca dell'Astronomia nella banda dei raggi X ancora inesplorata. Nonostante un vasto interesse testimoniato da un'estesa letteratura, il suo sviluppo è stato impedito dalla mancanza di dispositivi la cui sensibilità giustificasse l'inserimento di polarimetri X a bordo delle missioni spaziali moderne. Ciononostante nuovi strumenti basati sull'effetto fotoelettrico, capaci di risolvere le tracce dei fotoelettroni in un gas, offrono oggi la possibilità di colmare il divario tra aspettative teoriche e gli scarsi risultati ad oggi raggiunti: in questo ambito, uno dei progetti più avanzati è il Gas Pixel Detector (GPD), sviluppato in Italia dall'INFN di Pisa e dall'INAF/IASF di Roma. Questa tesi ha contribuito dell'inserimento del GPD a bordo delle prossime missioni in differenti aspetti. Innanzitutto è stata misurata la risposta dello strumento a radiazione polarizzata di alcuni keV, ovvero nell'intervallo energetico di massima sensibilità, grazie ad una sorgente di calibrazione basata sulla diffrazione di Bragg a circa 45 gradi. Quest'ultima, insieme a sorgenti radioattive e a tubi a raggi X, è stata montata su un sistema meccanico, progettato e costruito in modo tale che lo strumento in esame potesse essere spostato, inclinato e ruotato rispetto al fascio incidente. Questo rende disponibile una struttura per la completa caratterizzazione del GPD (e di altri strumenti): prime e preliminari misure di calibrazione sono state effettuate grazie ad essa e sono di seguito presentate. Infine ho calcolato la sensibilità del GPD nel caso in cui esso sia posto nel piano focale di telescopi progettati per le prossime missioni spaziali, come PolariX e IXO, e ho discusso ulteriori applicazioni del GPD e dei polarimetri fotoelettrici in generale. In particolare, sono presentati il possibile utilizzo di questi dispositivi come strumenti a grande campo di vista o e quello come rivelatori sensibili fino ad energie di decine di keV.<br>Polarimetry is the last unexplored branch of X-ray Astronomy. Despite a wide interest proved by an extended literature, its development has suffered the lack of instruments which could assure a sufficient sensitivity to justify the inclusion of X-ray polarimeters on-board modern missions. Nevertheless new devices based on photoelectric absorption, which are able to image the track of photoelectrons in a gas mixture, offer today the possibility to fill the gap between theoretical expectations and the current meagre results: in this regard, one of the most advanced project is the GPD (Gas Pixel Detector), developed in Italy by INFN of Pisa and INAF/IASF of Rome. This work of thesis contributed to the use of the GPD on-board future space missions from different points of view. The response of the instrument to polarized radiation at a few keV, namely in the energy range of maximum sensitivity, was measured thanks to a calibration source based on Bragg diffraction at nearly 45 degrees. This source, together with radioactive unpolarized sources and X-ray tubes, were also interfaced with a mechanical assembly which was designed and built allowing for the movement, the inclination and the rotation of the instrument with respect to the incident beam. This makes available a facility for the complete characterization of the GPD (and other instruments) and the first and preliminary systematic measurements of calibration are presented. I also derive the scientific performances of the GPD when the instrument is placed in the focal plane of realistic X-ray telescopes planned for future missions, such as PolariX or IXO, and discuss some further applications of the GPD and of the photoelectric polarimeters in general. In particular the possible use of these devices as large field of view instruments or at energies up to tens of keV are presented.

I have studied entropy profiles obtained in a sample of 24 X-ray objects at high redshift retrieved from the Chandra archive. I have discussed the scaling properties of the entropy S, the correlation between metallicity Z and S, the profiles of the temperature of the gas, Tgas, and performed a comparison between the dark matter 'temperature' and Tgas in order to constrain the non-gravitational processes which affect the thermal history of the gas. Furthermore I have studied the scaling relations between the X-ray quantities and Sunyaev Zel'dovich measurements. I have observed that X-ray laws are steeper than the relations predicted from the adiabatic model. These deviations from expectations based on self-similarity are usually interpreted in terms of feedback processes leading to non-gravitational gas heating, and suggesting a scenario in which the ICM at higher redshift has lower both X-ray luminosity and pressure in the central regions than the expectations from self-similar model. I have also investigated a Bayesian X-ray and Sunyaev Zel'dovich analysis, which allows to study the external regions of the clusters well beyond the volumes resolved with X-ray observations (1/3-1/2 of the virial radius), to measure the deprojected physical cluster properties, like temperature, density, entropy, gas mass and total mass up to the virial radius.

I have studied entropy profiles obtained in a sample of 24 X-ray objects at high redshift retrieved from the Chandra archive. I have discussed the scaling properties of the entropy S, the correlation between metallicity Z and S, the profiles of the temperature of the gas, Tgas, and performed a comparison between the dark matter 'temperature' and Tgas in order to constrain the non-gravitational processes which affect the thermal history of the gas. Furthermore I have studied the scaling relations between the X-ray quantities and Sunyaev Zel'dovich measurements. I have observed that X-ray laws are steeper than the relations predicted from the adiabatic model. These deviations from expectations based on self-similarity are usually interpreted in terms of feedback processes leading to non-gravitational gas heating, and suggesting a scenario in which the ICM at higher redshift has lower both X-ray luminosity and pressure in the central regions than the expectations from self-similar model. I have also investigated a Bayesian X-ray and Sunyaev Zel'dovich analysis, which allows to study the external regions of the clusters well beyond the volumes resolved with X-ray observations (1/3-1/2 of the virial radius), to measure the deprojected physical cluster properties, like temperature, density, entropy, gas mass and total mass up to the virial radius.

It is known that massive black holes have a profound effect on the evolution of galaxies, and possibly on their formation by regulating the amount of gas available for the star formation. However, how black hole and galaxies communicate is still an open problem, depending on how much of the energy released interacts with the circumnuclear matter. In the last years, most studies of feedback have primarily focused on AGN jet/cavity systems in the most massive galaxy clusters. This thesis intends to investigate the feedback phenomena in radio--loud AGNs from a different perspective studying isolated radio galaxies, through high-resolution spectroscopy. In particular one NLRG and three BLRG are studied, searching for warm gas, both in emission and absorption, in the soft X-ray band. I show that the soft spectrum of 3C33 originates from gas photoionized by the central engine. I found for the first time WA in 3C382 and 3C390.3. I show that the observed warm emitter/absorbers is not uniform and probably located in the NLR. The detected WA is slow implying a mass outflow rate and kinetic luminosity always well below 1% the L(acc) as well as the P(jet). Finally the radio--loud properties are compared with those of type 1 RQ AGNs. A positive correlation is found between the mass outflow rate/kinetic luminosity, and the radio loudness. This seems to suggest that the presence of a radio source (the jet?) affects the distribution of the absorbing gas. Alternatively, if the gas distribution is similar in Seyferts and radio galaxies, the M(out) vs rl relation could simply indicate a major ejection of matter in the form of wind in powerful radio AGNs.

It is known that massive black holes have a profound effect on the evolution of galaxies, and possibly on their formation by regulating the amount of gas available for the star formation. However, how black hole and galaxies communicate is still an open problem, depending on how much of the energy released interacts with the circumnuclear matter. In the last years, most studies of feedback have primarily focused on AGN jet/cavity systems in the most massive galaxy clusters. This thesis intends to investigate the feedback phenomena in radio--loud AGNs from a different perspective studying isolated radio galaxies, through high-resolution spectroscopy. In particular one NLRG and three BLRG are studied, searching for warm gas, both in emission and absorption, in the soft X-ray band. I show that the soft spectrum of 3C33 originates from gas photoionized by the central engine. I found for the first time WA in 3C382 and 3C390.3. I show that the observed warm emitter/absorbers is not uniform and probably located in the NLR. The detected WA is slow implying a mass outflow rate and kinetic luminosity always well below 1% the L(acc) as well as the P(jet). Finally the radio--loud properties are compared with those of type 1 RQ AGNs. A positive correlation is found between the mass outflow rate/kinetic luminosity, and the radio loudness. This seems to suggest that the presence of a radio source (the jet?) affects the distribution of the absorbing gas. Alternatively, if the gas distribution is similar in Seyferts and radio galaxies, the M(out) vs rl relation could simply indicate a major ejection of matter in the form of wind in powerful radio AGNs.

One of the most debated topics of modern extragalactic astrophysics is the co-evolution between the SMBHs sitting in the nuclei of the galaxies and their hosts. According to the most popular models of AGN and galaxies co-evolution, starburst galaxies, unobscured quasars and inactive (and quenched) galaxies represent the different phases of an evolutionary sequence reproducing the most massive galaxies observed in the Universe. In this context, gas flows in the form of energetic outflows are postulated to play a pivotal role in this process, given that they regulate both accretion and ejection of material onto and from the SMBH, acting on the fuel from which new stars form and stopping the host galaxy growth. In order to validate this AGN-galaxy binding nature, we investigate the kinematic and physical properties of ionized and atomic gas components of the ISM of well selected AGN-galaxy systems, obtained from ultraviolet, optical, near-infrared spectroscopic analysis, to unveil the presence of outflows. In particular, we propose and test different selection criteria to isolate sources at z ~2 in the predicted feedback phase, starting from multiwavelength information collected in the COSMOS field. As a complementary approach, we also collect a large sample of X-ray selected AGNs at z < 0.8 from the SDSS database to unveil the incidence of outflows and prove the crucial role of X-ray observations for the investigation of the feedback phase. We combine the ionized/neutral gas kinematic and physical conditions with SMBH accretion and host galaxy properties, recovered from multiwavelength spectral energy distribution analysis and X-ray spectroscopic investigation, to infer the general conditions regulating the feedback processes. We also discuss our results in the context of the more recent observational and theoretical clues.

In this thesis, I have investigated the evolution of the high-redshift (z > 3) AGN population by collecting data from some of the major Chandra and XMM-Newton surveys. The final sample (141 sources) is one of the largest selected at z> 3 in the X- rays and it is characterised by a very high redshift completeness (98%). I derived the spectral slopes and obscurations through a spectral anaysis and I assessed the high-z evolution by deriving the luminosity function and the number counts of the sample. The best representation of the AGN evolution is a pure density evolution (PDE) model: the AGN space density is found to decrease by a factor of 10 from z=3 to z=5. I also found that about 50% of AGN are obscured by large column densities (logNH > 23). By comparing these data with those in the Local Universe, I found a positive evolution of the obscured AGN fraction with redshift, especially for luminous (logLx > 44) AGN. I also studied the gas content of z < 1 AGN-hosting galaxies and compared it with that of inactive galaxies. For the first time, I applied to AGN a method to derive the gas mass previously used for inactive galaxies only. AGN are found to live preferentially in gas-rich galaxies. This result on the one hand can help us in understanding the AGN triggering mechanisms, on the other hand explains why AGN are preferentially hosted by star-forming galaxies.

In this thesis, I have investigated the evolution of the high-redshift (z > 3) AGN population by collecting data from some of the major Chandra and XMM-Newton surveys. The final sample (141 sources) is one of the largest selected at z> 3 in the X- rays and it is characterised by a very high redshift completeness (98%). I derived the spectral slopes and obscurations through a spectral anaysis and I assessed the high-z evolution by deriving the luminosity function and the number counts of the sample. The best representation of the AGN evolution is a pure density evolution (PDE) model: the AGN space density is found to decrease by a factor of 10 from z=3 to z=5. I also found that about 50% of AGN are obscured by large column densities (logNH > 23). By comparing these data with those in the Local Universe, I found a positive evolution of the obscured AGN fraction with redshift, especially for luminous (logLx > 44) AGN. I also studied the gas content of z < 1 AGN-hosting galaxies and compared it with that of inactive galaxies. For the first time, I applied to AGN a method to derive the gas mass previously used for inactive galaxies only. AGN are found to live preferentially in gas-rich galaxies. This result on the one hand can help us in understanding the AGN triggering mechanisms, on the other hand explains why AGN are preferentially hosted by star-forming galaxies.

Seyfert galaxies are the closest active galactic nuclei. As such, we can use them to test the physical properties of the entire class of objects. To investigate their general properties, I took advantage of different methods of data analysis. In particular I used three different samples of objects, that, despite frequent overlaps, have been chosen to best tackle different topics: the heterogeneous BeppoS AX sample was thought to be optimized to test the average hard X-ray (E above 10 keV) properties of nearby Seyfert galaxies; the X-CfA was thought the be optimized to compare the properties of low-luminosity sources to the ones of higher luminosity and, thus, it was also used to test the emission mechanism models; finally, the XMM–Newton sample was extracted from the X-CfA sample so as to ensure a truly unbiased and well defined sample of objects to define the average properties of Seyfert galaxies. Taking advantage of the broad-band coverage of the BeppoS AX MECS and PDS instruments (between ~2-100 keV), I infer the average X-ray spectral propertiesof nearby Seyfert galaxies and in particular the photon index (<Gamma>~1.8), the high-energy cut-off (<Ec>~290 keV), and the relative amount of cold reflection (<R>~1.0). Moreover the unified scheme for active galactic nuclei was positively tested. The distribution of isotropic indicators used here (photon index, relative amount of reflection, high-energy cut-off and narrow FeK energy centroid) are similar in type I and type II objects while the absorbing column and the iron line equivalent width significantly differ between the two classes of sources with type II objects displaying larger absorbing columns. Taking advantage of the XMM–Newton and X–CfA samples I also deduced from measurements that 30 to 50% of type II Seyfert galaxies are Compton thick. Confirming previous results, the narrow FeK line is consistent, in Seyfert 2 galaxies, with being produced in the same matter responsible for the observed obscuration. These results support the basic picture of the unified model. Moreover, the presence of a X-ray Baldwin effect in type I sources has been measured using for the first time the 20-100 keV luminosity (EW proportional to L(20-100)^(−0.22±0.05)). This finding suggests that the torus covering factor may be a function of source luminosity, thereby suggesting a refinement of the baseline version of the unifed model itself. Using the BeppoSAX sample, it has been also recorded a possible correlation between the photon index and the amount of cold reflection in both type I and II sources. At a first glance this confirms the thermal Comptonization as the most likely origin of the high energy emission for the active galactic nuclei. This relation, in fact, naturally emerges supposing that the accretion disk penetrates, depending to the accretion rate, the central corona at different depths (Merloni et al. 2006): the higher accreting systems hosting disks down to the last stable orbit while the lower accreting systems hosting truncated disks. On the contrary, the study of the well defined X–C f A sample of Seyfert galaxies has proved that the intrinsic X-ray luminosity of nearby Seyfert galaxies can span values between 10^(38−43) erg s^−1, i.e. covering a huge range of accretion rates. The less efficient systems have been supposed to host ADAF systems without accretion disk. However, the study of the X–CfA sample has also proved the existence of correlations between optical emission lines and X-ray luminosity in the entire range of L_(X) covered by the sample. These relations are similar to the ones obtained if high-L objects are considered. Thus the emission mechanism must be similar in luminous and weak systems. A possible scenario to reconcile these somehow opposite indications is assuming that the ADAF and the two phase mechanism co-exist with different relative importance moving from low-to-high accretion systems (as suggested by the Gamma vs. R relation). The present data require that no abrupt transition between the two regimes is present. As mentioned above, the possible presence of an accretion disk has been tested using samples of nearby Seyfert galaxies. Here, to deeply investigate the flow patterns close to super-massive black-holes, three case study objects for which enough counts statistics is available have been analysed using deep X-ray observations taken with XMM–Newton. The obtained results have shown that the accretion flow can significantly differ between the objects when it is analyzed with the appropriate detail. For instance the accretion disk is well established down to the last stable orbit in a Kerr system for IRAS 13197-1627 where strong light bending effect have been measured. The accretion disk seems to be formed spiraling in the inner ~10-30 gravitational radii in NGC 3783 where time dependent and recursive modulation have been measured both in the continuum emission and in the broad emission line component. Finally, the accretion disk seems to be only weakly detectable in rk 509, with its weak broad emission line component. Finally, blueshifted resonant absorption lines have been detected in all three objects. This seems to demonstrate that, around super-massive black-holes, there is matter which is not confined in the accretion disk and moves along the line of sight with velocities as large as v~0.01-0.4c (whre c is the speed of light). Wether this matter forms winds or blobs is still matter of debate together with the assessment of the real statistical significance of the measured absorption lines. Nonetheless, if confirmed, these phenomena are of outstanding interest because they offer new potential probes for the dynamics of the innermost regions of accretion flows, to tackle the formation of ejecta/jets and to place constraints on the rate of kinetic energy injected by AGNs into the ISM and IGM. Future high energy missions (such as the planned Simbol-X and IXO) will likely allow an exciting step forward in our understanding of the flow dynamics around black holes and the formation of the highest velocity outflows.

Seyfert galaxies are the closest active galactic nuclei. As such, we can use them to test the physical properties of the entire class of objects. To investigate their general properties, I took advantage of different methods of data analysis. In particular I used three different samples of objects, that, despite frequent overlaps, have been chosen to best tackle different topics: the heterogeneous BeppoS AX sample was thought to be optimized to test the average hard X-ray (E above 10 keV) properties of nearby Seyfert galaxies; the X-CfA was thought the be optimized to compare the properties of low-luminosity sources to the ones of higher luminosity and, thus, it was also used to test the emission mechanism models; finally, the XMM–Newton sample was extracted from the X-CfA sample so as to ensure a truly unbiased and well defined sample of objects to define the average properties of Seyfert galaxies. Taking advantage of the broad-band coverage of the BeppoS AX MECS and PDS instruments (between ~2-100 keV), I infer the average X-ray spectral propertiesof nearby Seyfert galaxies and in particular the photon index (<Gamma>~1.8), the high-energy cut-off (<Ec>~290 keV), and the relative amount of cold reflection (<R>~1.0). Moreover the unified scheme for active galactic nuclei was positively tested. The distribution of isotropic indicators used here (photon index, relative amount of reflection, high-energy cut-off and narrow FeK energy centroid) are similar in type I and type II objects while the absorbing column and the iron line equivalent width significantly differ between the two classes of sources with type II objects displaying larger absorbing columns. Taking advantage of the XMM–Newton and X–CfA samples I also deduced from measurements that 30 to 50% of type II Seyfert galaxies are Compton thick. Confirming previous results, the narrow FeK line is consistent, in Seyfert 2 galaxies, with being produced in the same matter responsible for the observed obscuration. These results support the basic picture of the unified model. Moreover, the presence of a X-ray Baldwin effect in type I sources has been measured using for the first time the 20-100 keV luminosity (EW proportional to L(20-100)^(−0.22±0.05)). This finding suggests that the torus covering factor may be a function of source luminosity, thereby suggesting a refinement of the baseline version of the unifed model itself. Using the BeppoSAX sample, it has been also recorded a possible correlation between the photon index and the amount of cold reflection in both type I and II sources. At a first glance this confirms the thermal Comptonization as the most likely origin of the high energy emission for the active galactic nuclei. This relation, in fact, naturally emerges supposing that the accretion disk penetrates, depending to the accretion rate, the central corona at different depths (Merloni et al. 2006): the higher accreting systems hosting disks down to the last stable orbit while the lower accreting systems hosting truncated disks. On the contrary, the study of the well defined X–C f A sample of Seyfert galaxies has proved that the intrinsic X-ray luminosity of nearby Seyfert galaxies can span values between 10^(38−43) erg s^−1, i.e. covering a huge range of accretion rates. The less efficient systems have been supposed to host ADAF systems without accretion disk. However, the study of the X–CfA sample has also proved the existence of correlations between optical emission lines and X-ray luminosity in the entire range of L_(X) covered by the sample. These relations are similar to the ones obtained if high-L objects are considered. Thus the emission mechanism must be similar in luminous and weak systems. A possible scenario to reconcile these somehow opposite indications is assuming that the ADAF and the two phase mechanism co-exist with different relative importance moving from low-to-high accretion systems (as suggested by the Gamma vs. R relation). The present data require that no abrupt transition between the two regimes is present. As mentioned above, the possible presence of an accretion disk has been tested using samples of nearby Seyfert galaxies. Here, to deeply investigate the flow patterns close to super-massive black-holes, three case study objects for which enough counts statistics is available have been analysed using deep X-ray observations taken with XMM–Newton. The obtained results have shown that the accretion flow can significantly differ between the objects when it is analyzed with the appropriate detail. For instance the accretion disk is well established down to the last stable orbit in a Kerr system for IRAS 13197-1627 where strong light bending effect have been measured. The accretion disk seems to be formed spiraling in the inner ~10-30 gravitational radii in NGC 3783 where time dependent and recursive modulation have been measured both in the continuum emission and in the broad emission line component. Finally, the accretion disk seems to be only weakly detectable in rk 509, with its weak broad emission line component. Finally, blueshifted resonant absorption lines have been detected in all three objects. This seems to demonstrate that, around super-massive black-holes, there is matter which is not confined in the accretion disk and moves along the line of sight with velocities as large as v~0.01-0.4c (whre c is the speed of light). Wether this matter forms winds or blobs is still matter of debate together with the assessment of the real statistical significance of the measured absorption lines. Nonetheless, if confirmed, these phenomena are of outstanding interest because they offer new potential probes for the dynamics of the innermost regions of accretion flows, to tackle the formation of ejecta/jets and to place constraints on the rate of kinetic energy injected by AGNs into the ISM and IGM. Future high energy missions (such as the planned Simbol-X and IXO) will likely allow an exciting step forward in our understanding of the flow dynamics around black holes and the formation of the highest velocity outflows.

The study of the X-ray spectra of accreting compact objects such as neutron stars, and in particular the emission and absorption features common in the X-ray systems, provides a powerful tool to investigate the innermost parts of the accretion disc and the regions close to the compact object. The first part of my work concerned the analysis of the spectra, of two bright, persistent accreting low-mass X-binaries classified as atoll sources: GX 3+1 and GX 9+9. Broad emission features are shown in the spectrum of the first source; on the contrary, the second one shows the total absence of the reflection components and different scenarios have been proposed to explain this unexpected lack. The second part of this work regarded the HERMES satellite: a prototype modular detector for photons in the energy band from 4 keV to 1 MeV. The ensemble of these detectors represents an extraordinary instrument to estimate, using the trilateration method, he position of the Gamma Ray Burst with unprecedented accuracy. A first evaluation of these delays, computed through simulations, will be provided.

The purpose of this work has been the study of Radio Emitting X-Ray Binaries (REXRBs) both from the observational and theoretical approaches. <br/><br/>We have mainly concentrated in the analysis of their properties inferred from their emission in radio and X-ray wavelengths. Our observational contribution has been done specially at different centimetric and millimetric wavelengths, although contrast with other bands of the electromagnetic spectrum has been always considered. <br/><br/>From the about 25 REXRBs detected, we have directly observed and analyzed the sources LS 1+61 °303 and Cygnus X-3, both radiostars known for they highly variable radio emission events. <br/><br/>The most remarkable characteristic of LS I+61°303 is the recurrent display of radio outbursts that show complex light curve profiles with a period similar to 26.5 days. It is noticeable that, with the exception of pulsars, LS I+61°303 and Cygnus X-l, are the only two known periodic radio sources. Cygnus X-3 is also well known for the exhibition of strong radio flares. The system spends most of the time in quiescent state, at a level of few hundreds of mJv. It normally exhibits variability in time scales of hours suspected of having periodical or quasi periodical trends. When an outburst occurs the source reaches peak flux densities of tens of Jv at centimetric wavelengths, which converts it during some days in one of the brightest radio source of the sky. Cygnus X-3 strong flares are irregularly spaced with a mean recurrence interval of ~120 days. <br/><br/>The flaring events are believed to involve the ejection and acceleration of ionized plasma clouds (plasmons) responsible for the radio emission via synchrotron radiation. <br/><br/>The reasons for selecting this two sources as the objects to use in our study are mainly related to their flaring behaviour, and they can be summarized in the following points: <br/><br/>· Due to their outbursting properties they are intrinsically interesting as independent sources among the REXRBs. <br/><br/>· The sudden release of relativistic particles that provokes the flaring radio emission can help to better understand the origin of such particles and conditions of the system at the epochs of the outburst occurrence. That can be extrapolated to non-flaring radio emitters. <br/><br/>· In concrete, the periodical behaviour of LS I+61°303 (the only REXRB of such characteristic in the northern hemisphere) allows to predict the phases when the source is in active state. Such predictability facilitates the scheduling of observations, depending on whether we are interested to perform the experiment in quiescence or in active state. It also permits to coordinate different instruments for multiple energy band monitoring of the source, normally preferred when it is in a high emission state. <br/><br/>· The periodical behaviour itself is also very interesting. It is presumably related to the orbital motion of the system, and onset of outburst are suspected to occur near the periastron passage. Such behaviour can indicate a very eccentric orbit for the binary system LS I+61°303. The confirmation of such issues can provide clues for physical parameters also for more steady radio emitting REXRBs. as well as for non radio emitting X-ray binaries. <br/><br/>· The observation of the temporal and spectral evolution of a radio flaring event, as well as the source structure at different spatial scales during it, can provide us with useful information of the mass of the particles involved, the energetic evolution of such radiating particles, the velocities acquired by the possible radio emitting bulk and its expansion rates, and the scenario in which the source is embebbed (geometry and strength of the magnetic field, surrounding non relativistic material, etc). <br/><br/>· And finally, the study of these two objects is a continuation of the work per-formed by members of the research group where the author is integrated. They disposed of previous observations of these objects, as well as experience in radio single dish, and array observations and wide knowledge of this REXRBs and the different physical models that could describe them. <br/><br/>This report is structured in 11 chapters, being the bulk of them (with the exception of the present one) distributed in three main differentiated parts. Each part is a compilation of the work performed to study the next different aspects of REXRBs and in particular of our selected objects: <br/><br/>Part I - Highly energetic processes in REXRBs emission: In Chapter 2 we develop a theoretical model of the radio and X-ray emission arisen from a plasmon of relativistic particles within a REXRB scenario. In Chapter 3 we apply this model to observations of LS I+61°303 and Cygnus X-3. <br/><br/>Part II - Radio images: We analyze sub-arsecond, arcsecond and arcminute images we have performed from different observation campaigns of LS I+61°303 and Cygnus X-3. They are described in Chapters 4, 5 and 6. <br/><br/>Part III - Search for periodic behaviour in the radio and X-ray light curves: <br/>After a brief description of statistical methods to look for a periodic signal from a data set, in Chapter 7 we apply them to analyze the long and short time scales of the variable behaviour of the target sources. Chapter 8 is devoted to look for the instabilities in the ~ 26.5 days radio period of LS I+61°303, and Chapter 9 to analyze its 'micro-flares' repetitive behaviour at certain phases. Chapter 10 is as well devoted to periodicities of LS I+61°303, but in this case in the X-ray domain, and, finally, Chapter 11 analyzes the hour scale fluctuations observed in Cygnus X-3.<br><i>TÍTOL DE LA TESI:<br/><br/>"Les estrelles binàries de raigs X amb emissió ràdio LS I+61°303 i Cygnus X-3" <br/><br/>TEXT: <br/><br/>Les binàries o estrelles dobles físiques són sistemes formats per dues estrelles lligades gravitatòriament. Avui en dia sabem que aquest és un fenomen molt comú a la Galàxia, on aproximadament la meitat de les estrelles s'han format donant lloc a aquesta mena de sistemes. Històricament, el seu estudi ha proporcionat molta informació sobre la física estel·lar. Per exemple, fins no fa gaire els sistemes binaris eren la millor eina disponible per a la mesura directa de la massa dels estels. Ha estat també gràcies a observacions de sistemes binaris contenint púlsars que s'ha obtingut la verficació de previsions de la teoria de la Relativitat General. <br/><br/>Durant gran part de la seva vida, les components d'un sistema binari orbiten a distancias comparativament més grans que les dimensions del volum on domina la influència gravitatòria de cadascuna de les components (lòbul de Roche). Durant aquest temps, les estrelles del sistema evolucionen de la mateixa manera que ho farien si es trobessin aïllades. No obstant, les diferents etapes de l'evolució estel·lar per les que, inexorablement, cadascuna de les components haurà de passar segons la seva massa, poden fer que els paràmetres físics del sistema (massa, radi, semieix de l'òrbita, vent estel·lar, etc.) varîin apreciablement. A conseqüència d'aquests canvis, es poden produir situacions en què el radi d'una de les components es faci comparable, o fins i tot l'excedeixi, al tamany característic del seu lòbul de Roche. A partir d'aquí, és possible que s'estableixi una tranferència de massa entre les components del sistema. Altrament, aquesta transferència també poder ésser deguda al vent estel·lar. La captura o acreció d'una fracció d'aquesta massa, perduda pel-la component "donant", per part de la seva companya pot donar lloc a fenòmens capaços de modificar apreciablement les propietats globals del sistema, a l'hora que també pot generar un ampli ventall de fenòmens observacionals que es manifesten en gairebé tot l'espectre electromagnètic. <br/><br/>És molt variada la "fauna" de sistemes binaris interactuants segons sigui el mecanisme concret de transferència de massa i el tipus d'estrelles components. En aquest treball ens ocupem en particular de les anomenades binàries de raigs X, de les quals s'en coneixen actualment més d'un centenar. Entre elles, però, centrem la nostra atenció en les que, a més, han estat detectades en ones de ràdio, a les quals ens referirem simplement per brevetat com a ràdio binàries X. Aquesta subclasse de les binàries de raigs X, amb emissió ràdio associada, és certament força heterogènia i fins avui s'en coneixen de l'ordre de 20 casos. A títol d'exemple, aquests inclouen sistemes com Cygnus X-l, el primer canditat a forat negre, LS I+61°303 i Circinus X-l, les dues úniques ràdiofonts del cel clarament periòdiques a part dels pulsáis, i Cygnus X-3, famosa per les seves erupcions ràdio on la lluminositat augmenta fins a tres ordres de magnitud en poc més d'un dia. L'interés astrofísic d'aquestes ràdio binàries X es podia entendre, fins no fa gaire, només en que molts dels processos físics que hi tenen lloc són versions a escala reduïda del que succeix en algunes galàxies actives i quàsars. Això ja és de per sí remarcable doncs, donat que totes elles són objectes pertanyents a la nostra Galàxia i per tant relativament propers, les tècniques observacionals de la Interferometria de Base Molt Llarga ( Very Long Base-line Interferometry o VLBI) ens permeten assolir-hi unes ressolucions espaials (~ 1 AU) impensables en una font extragalàctica. D'altra banda, les escales temporals d'emissió en ràdio binàries són relativament curtes (mesos o dies) i, en ocasions, fins i tot periòdiques d'acord amb el període orbital del sistema. Això facilita enormement preveure les èpoques d'observació en que hom espera més activitat. <br/><br/>D'altra banda, aquest tipus d'objectes s'han beneficiat considerablement dels darrers avenços en les tècniques observacionals, tant des de terra com des de l'espai, que actualment fan possible realitzar observacions astronòmiques en tots els dominis de l'espectre electromagnètic. Això és particularment interessant quan s'aplica a astres que siguin détectables a la vegada en més d'un aquests dominis, com és el cas de les ràdio binàries X. Aleshores, les observacions multi-espectrals representen una eina molt important per a contrastar l'elaboració de models teòrics capaços de reproduir, de manera unificada, el comportament observat a longituds d'ona molt diferents (ràdio, infrarroig, òptic, ultraviolat, raigs X i raigs Gamma). <br/><br/>Més recentment, però, s'hi ha afegit un nou punt d'interés després del descobri-ment de diverses fonts de raigs X durs (> 40 keV) en la direcció del centre galàctic. Aquests nous objectes presenten una contrapartida ràdio constituïda per una font central compacta i variable de la qual emana un jet bipolar. La interpretació més probable és que es tracta de ràdio binàries X on la component compacta és un forat negre amb un disc d'acreció, perpendicularment al qual s'ejecta plasma altament relativista en forma de jet col.limat. Una d'elles, GRS1915+105, ha resultat ser la primera ràdiofont coneguda de la Galàxia que ejecta matèria a velocitats aparent-ment superlumíniques. <br/><br/>En general, les binàries de raigs X són sistemes formats per un estel normal i un objecte compacte i col·lapsat. Aquest darrer és habitualment un estel de neutrons però, en ocasions, pot tractar-se d'un forat negre. El nom de binàries de raigs X es justifica pels processos d'acreció de matèria sobre l'objecte compacte, els quals originen intenses emissions en aquest domini espectral. Segons la massa de l'estel normal, que actua com a donant de matèria, les binàries de raigs X es classifiquen a grans trets en d'alta massa o de baixa massa, sense que es coneguin exemples intermitjos. La component normal de les binàries massives pertany típicament a tipus espectrals primerencs mentre que, en les poc massives, aquesta és de tipus espectral més tardà. <br/><br/>Suposem que la component compacta del sistema binari acreta matèria a un cert ritme Macc- Aleshores, la lluminositat del sistema Lacc es pot estimar suposant que tota l'energia cinètica del material acretat s'allibera en forma de radiació. </I>

X-ray binaries are binary stars composed of a compact object (a black hole, a neutron star) accreting matter from a companion star. These sources can be considered perfect astrophysical laboratories to test our knowledge of, e.g., General Relativity and Magneto-Hydrodynamics. Accretion is the key phenomenon characterizing these systems, but it is not always completely efficient. In many systems, ejections of matter are also observed, e.g. in the form of jets and winds, or also suggested, e.g. to explain the observed strong orbital expansion of a number of systems. Furthermore accretion and ejection seems to be somehow interconnected but the nature of this correlation is not completely clear. The purpose of this thesis is the study of a number of cases where the accretion is imperfect and mass losses have to be taken into account to correctly model the physical properties of the binaries. In the first of the featured projects, I focus on the spectral study of the accretion flow in the Neutron Star (NS) Low Mass X-ray Binary (LMXB) 1RXS J180408.9-342058, an intriguing system which in the past exhibited “very faint” phases of activity. I performed a spectral analysis of data collected by different X-ray telescopes, i.e. INTEGRAL, Swift and NuSTAR, The study led to several interesting results, in particular the observation of the intermediate spectral state, hard to catch in NS LMXBs because very short-lived, and new constraints on the nature of the companion star, which exclude the hypothesis of a helium dwarf companion as suggested in the past. The second project presents a systematic study of (almost) all known Accreting Millisecond X-ray Pulsars (AMXPs), i.e. LMXBs hosting an X-ray pulsar spinning at millisecond periods, with the aim of looking for indications of non-conservative mass-transfer in this class. Comparing this observed luminosity averaged over twenty years with the one expected from the theory in a conservative scenario, I found that over a sample of 19 sources, around one half of it shows indications for mass losses. The third project in this thesis is dedicated to jets, the most known form of mass ejection in X-ray binaries. Jets are characterized by flat radio-to-mid-IR spectra, which have been modelled in the last few decades using the Internal Shocks model ISHEM. The basic idea of this model consists in using the observed X-ray variability as a proxy for the fluctuations of the Lorentz factor in the ejected shells along the jet. I applied the model on the multi-wavelength data set of the NS LMXB 4U 0614+091. I found that ISHEM describes satisfactorily the data only in two cases: using the X-ray variability but in non-conical geometry or either in conical geometry but using flicker noise instead of the X-ray variability. The final project of my thesis aims at testing a unified accretion-ejection model to the Black Hole LMXB MAXI J1820+070. The model considers the accretion flow in X-ray binaries as two-fold, comprising a truncated geometrically thin disk far from the Black Hole and a so-called jet emitting disk serving as the base of the jet close to the Black Hole. Interestingly, the model allows not only to describe the X-rays data, but also to predict the radio power emitted by the jet. In order to test the model, I used X-rays data from Swift and NuSTAR. The preliminary results of the spectral fitting suggest that the model is indeed effective in describing the observed X-ray spectra. Furthermore, the analysis reveals the need for describing the reflection spectrum with two reflection components instead of one: the origin of such intriguing component, if confirmed, will be object of future investigations.

Lo scopo di questo elaborato è la descrizione dei principi su cui si basano i telescopi ottici, radio e x nell'osservazione dei corpi celesti, la loro meccanica di funzionamento e le relative differenze. Un telescopio è uno strumento fisico volto a raccogliere radiazione proveniente da oggetti molto distanti in una grande regione di cielo facendola convogliare su un'area ristretta. Questo avviene tramite l'utilizzo ad esempio di specchi, lenti o antenne, in modo da ottenere la massima risoluzione possibile. I fotoni raccolti vengono poi fatti convogliare in dei rilevatori come un obiettivo, una lastra fotografica o una CCD che elaborando la radiazione forniscono l'immagine.

Gamma-ray binaries are extreme systems that produce non-thermal emission from radio to very-high-energy (above TeV) gamma rays, with the energy output in the spectral energy distribution (SED) dominated by the MeV–GeV photons. Their broadband emission is usually modulated by the orbital cycle of the system, which suggests that the physical conditions are also periodic and reproducible. The diversity of systems, together with the reproducibility of the conditions within each system, makes gamma-ray binaries excellent physical laboratories in which high energy particle acceleration, diffusion, absorption, and radiation mechanisms can be explored. Nevertheless, the number of known gamma-ray binaries is still very limited, and only a six binary systems have been classified as gamma-ray binaries. These systems produce outflows of relativistic particles emitting synchrotron radio emission that extend up to several astronomical units, which correspond to projected angular scales of a few milliarcseconds (mas) at typical distances of 2-3 kpc. Very Long Baseline Interferometry (VLBI) provide mas resolution and therefore can be used to directly see this radio outflow. In this thesis we present VLBI observations of five of the six gamma-ray binaries known. We have revealed for the first time the radio structure of two gamma-ray binaries, and found periodic changes in the structure of other two. Based on these results, we have established the basic properties and behaviour of the radio emission of gamma-ray binaries on AU scales, and we have contributed to find characteristics that are common to all of them.<br>En los últimos años se ha producido una mejora significativa de los instrumentos que permiten observar fenómenos astrofísicos en rayos gamma de alta y muy alta energía. Gracias a estos avances, se ha podido detectar emisión de rayos gamma en sistemas binarios. Tan sólo seis sistemas binarios han sido clasificados como estrellas binarias de rayos gamma (tres de ellos aún son candidatos). Estos sistemas producen chorros de material relativista que a su vez producen grandes cantidades de energía en todo el espectro electromagnético, desde ondas radio hasta emisión de rayos gamma hasta energías del teraelectronvolt (TeV). Estos chorros se desplazan a alta velocidad produciendo estructuras en escalas de varias unidades astronómicas (UA). Estas estructuras pueden ser observadas directamente mediante técnicas de interferometría radio de muy larga línea de base (VLBI). En esta tesis nos centramos en el estudio de las propiedades morfológicas y astrométricas de binarias de rayos gamma observadas mediante VLBI. De las seis binarias conocidas, se han observado cinco de ellas. Los resultados principales son los siguientes. Se ha detectado estructura extendida en escalas de 120 UA en el sistema binario PSR B1259-63. Esta ha sido la primera evidencia observacional de que púlsares jóvenes no acretantes interaccionando con estrellas jóvenes pueden producir emisión radio extendida. Se ha descubierto que la emisión del sistema LS 5039 muestra cambios periódicos en su morfología, que son estables en escalas de varios años. También se ha determinado el movimiento propio de este sistema y se ha obtenido su trayectoria galáctica en el pasado. Se ha confirmado que el sistema LS I +61 303 muestra variabilidad orbital periódica, aunque presenta cambios significativos en ciertas fases orbitales. Se ha encontrado un desplazamiento del pico de la emisión a varias frecuencias, así como un cambio en sus posiciones relativas. Se ha descubierto emisión extendida procedente de la fuente de rayos gamma HESS J0632+057, y se ha confirmado inequívocamente su asociación con el sistema binario MWC 148. Por último, no se ha encontrado contrapartida radio a la fuente de rayos gamma AGL 2241+4454, cuya contrapartida óptica ha sido propuesta en el sistema MWC 656. Estos resultados permiten sentar las bases de la estructura en escalas de varias UA de los sistemas binarios de rayos gamma, así como su comportamiento en función de la fase orbital. Las características comunes halladas en estos sistemas ha permitido encontrar enlaces observacionales entre estos sistemas, dando consistencia a este particular grupo de estrellas que presentan emisión en rayos gamma.

I Nuclei Galattici Attivi (AGN) sono tra le sorgenti più luminose dell’Universo e sono osservati in circa 10-20% delle galassie nell’Universo locale. Il meccanismo di generazione dell’energia di queste sorgenti è l’accrescimento di materia su un buco nero supermassivo (SMBH). Secondo il paradigma del Modello Unificato (UM) il SMBH è circondato da un assorbitore uniforme e anisotropo di gas e polveri, con geometria toroidale, che si estende da 1pc a 100pc. Anche se a livello generale l’UM è corretto, lavori recenti hanno evidenziato la necessità di formulare modelli alternativi per spiegare ad esempio la presenza di molteplici assorbitori disomogenei, a diverse distanze dal SMBH e in diversi stati fisici. Tra le argomentazioni in conflitto con l’UM vi è la variabilità spesso osservata negli spettri X degli AGN. Quando non deriva da una variazione intrinseca dell’emissione nucleare, essa implica che l’assorbitore è ‘’clumpy’’ e non è uniforme. Vari studi hanno anche mostrato che l’assorbitore non è sempre neutro: sono stati osservati outflow di grandi quantità di materiale parzialmente ionizzato (noti come “warm absorbers”) con velocità di outflow tra 100 e 1000 km/s, in circa il 50% degli AGN di tipo 1. Un’ulteriore complessità è rappresentata dalle osservazioni di assorbitori altamente ionizzati con alte velocità di outflow (vout>3000 km/s) noti come “ultrafast outflows”. La mia tesi si occupa della variabilità di assorbimento osservata negli spettri X degli AGN, fondamentale per testare le dimensioni, la struttura e la composizione della materia circumnucleare. In dettaglio, ho analizzato lo spettro X di due AGN, NGC 454E e Mrk 348, per cui sono disponibili dati dei satelliti Swift-BAT, Suzaku e XMM-Newton. NGC 454E è una galassia di Seyfert 2, parte di un sistema di galassie interagenti, a z=0.0122. Confrontando gli spettri di Suzaku e XMM, con distanza temporale di 6 mesi, ho notato un forte cambiamento della curvatura spettrale tra 3 keV e 6 keV. La sua origine è riconducibile a una variazione importante della densità colonnale dell’assorbitore neutro, che ha permesso di classificare NGC 454E come “changing-look AGN”. Inoltre, lo spettro di XMM mostra segni della presenza di un assorbitore ionizzato, assente durante l’osservazione di Suzaku, con un limite superiore sulla distanza di circa 0.001 pc. Mrk 348 è una galassia classificata come Seyfert 2, a z=0.015. Il suo spettro è caratterizzato da tre diversi assorbitori, uno neutro e due ionizzati, con diverse densità e stati di ionizzazione, ma con circa la stessa velocità di outflow (∼0.05c). Le distanze degli assorbitori ionizzati dal SMBH sono probabilmente differenti, in quanto i limiti superiori sono trovati essere di 0.026pc per l’assorbitore di più alta ionizzazione e 2.72pc per quello di media ionizzazione. Anche Mrk 348 presenza una variazione della densità colonnale tra Suzaku e XMM, sia per l’assorbitore neutro che per quello di media ionizzazione. Questi studi costituiscono un passo in avanti per definire un modello alternativo all’UM, per cui ci sono sempre più prove che la regione circumnucleare è composta da diversi assorbitori, a diverse distanze e stati di ionizzazione.<br>Active Galactic Nuclei (AGN) are among the most energetic sources in the Universe, and they are observed in about 10-20% of the galaxies in the local Universe. Their energy generation mechanism is accretion of matter on a central Supermassive Black Hole (SMBH). According to the most accredited paradigm, the Unified Model (UM), the SMBH is surrounded by an anisotropic uniform absorber of gas and dust, with a toroidal geometry, extending from 1 pc to 100 pc. Even if on a basic level the UM is correct, the need for alternative models was highlighted by the detection of several dishomogeneous absorbers, located at different distances from the SMBH, and at different physical states. Among the arguments in conflict with the UM there is the variability often observed in the X-ray spectra of AGN. When not resultant from an intrinsic variation of the nuclear emission, it implies that the underlying absorber is clumpy and not uniform. Many studies also showed that, besides being clumpy, the absorbing material is not always completely neutral. Significant amounts of outflowing partially-ionized material have been detected in about ∼50% of Type 1 AGN. These ionized absorbers are called “warm absorbers” and have outflow velocities in the range ∼100−1000 km/s. Furthermore, highly ionized absorbers with high outflow velocities (vout>3000 km/s, “ultrafast outflows”) have been observed. This thesis focuses on observations of absorption variability in the X-ray spectra of AGN, as a useful “tool” to test the size, structure and composition of the circumnuclear medium. In detail, I analysed the X-ray spectra of two AGN, NGC 454E and Mrk 348, for which X-ray data from Swift-BAT, Suzaku and XMM-Newton were available. NGC 454E is a red elliptical Seyfert galaxy in an interacting system, at z=0.0122. When comparing the Suzaku and XMM-Newton spectra, observed with 6 months delay, I noticed a dramatic change in the spectral curvature between 3 keV and 6 keV. The origin of the variability was due to a strong change of the column density of a neutral absorber, that allowed to classify NGC 454E as a “changing-look AGN”. Moreover, the XMM-Newton spectrum showed clear signatures of the presence of an ionised absorber, absent during Suzaku observation. An upper limit on its distance was inferred to be ∼0.001 pc. Mrk 348 is a bright Compton-thin Seyfert 2 galaxy at z=0.015. I found that its spectrum is characterized by three different absorbers, one neutral and two ionised, with different densities and ionisation states, but with approximately the same outflow velocity (∼0.05c). The distances of the ionised absorbers from the central emitting region are likely to be different, indeed the upper limits I find are 0.026 pc for the highly ionised absorber, and 2.72 pc for the mildly ionised one. A variation of the column density is present also for Mrk 348, both for the neutral absorber and for the mildly ionized. Both these studies are a step forwards in the definition of an alternative model to the UM, for which more and more evidences are being collected, denoting that the absorbing material is composed by multiple absorbers at different distances, densities and ionization states.

The study of high-resolution X-ray spectra of neutron star low-mass X-ray binaries (LMXBs) allows the investigation of the innermost parts of the accretion disk and immediate surroundings of the compact object. The weak magnetic eld of old neutron stars present in such systems allows the accretion disk to approach very close to the compact object, like in black hole X-ray binaries. Using data from X-ray satellites such as XMM-Newton, RXTE, and BeppoSAX, I studied the reection component in two neutron star LMXBs: MXB 1728-34 and 4U 1735-44. I showed that the iron line at 6:4

This PhD thesis explores the role of low energy protons, the so-called ‘soft protons’, as a component of the background in view of the future ESA’s X-ray mission Athena. As a matter of fact, a high level of soft proton flux at the focal plane of Athena can adversely affect the scientific goals of the mission. To prevent this, a correct estimate of the soft proton flux expected at the focal plane of the satellite is fundamental. Such an estimate can be achieved only if the reflectivity of soft protons from the optics is well understood, with efforts on both the experimental and the theoretical sides. To this aim, I applied the model of reflectivity of particles at grazing incidence proposed by Remizovich et al. (1980), under the non-elastic approximation, to the experimental measurements of proton scattering at low incident angles from XMM-Newton and eROSITA mirror samples. The mismatch between the model and the experimental data led me to create a new analytical semi-empirical model, where the parameter σ enclosing the micro-physics of the interaction between the protons and the mirror lattice is directly derived by fitting the data. This new model gives a more accurate estimate of the scattering efficiency and energy loss distributions, but depends on the specific materials eROSITA and XMM-Newton are made of. For the model to be applied to Athena, new experimental data on Athena’s optics, the Silicon Pore Optics (SPO), are necessary. These new data were acquired during dedicated experimental campaigns carried out by the Institut für Astronomie und Astrophysik of the University of Tübingen. The experiment consisted in measuring the scattering of low energy protons at grazing incidence from an Athena SPO sample, at two different incident energies, ∼470 keV and ∼170 keV, and at four different incident angles, 0.6°, 0.8°, 1.0°, 1.2°. The new data are consistent, within the error bars, with the data from the eROSITA mirror sample, so that the same model can be used to estimate the scattering efficiency of SPO. A more accurate model can be built from a fit of the new data sets, provided that energy loss measurements are retrieved from the raw data. The new semi-empirical model can be implemented in a ray-tracing code to build a specific response matrix for protons. The construction of a proton response matrix is a 2-years project that falls within the AHEAD2020 activities, in view of the launch of Athena. The project foresees the construction of a proton response matrix for XMM-Newton as a reliable tool for the deconvolution of observed soft protons spectra. If the validation of this response matrix is successful, then the same procedure can be used to produce an analogous proton response matrix for Athena. In this framework, I performed a Geant4 simulation of the interaction of soft protons with the focal plane of XMM-Newton, consisting in a detailed representation of the 7 CCDs of the MOS camera, the filters, and the proton shields surrounding the focal plane assembly. The coupling of the Geant4 simulation with the output of the aforementioned ray-tracing will bring to the proton response matrix for XMM-Newton. To reach a round research profile, I also analysed observational X-ray data from two binary X-ray sources, which represent ideal cases to test to what extent soft protons can affect the quality of observational data. The low level of background required for Athena will improve the knowledge we have of these systems and will enhance advanced studies for a wider sample of X-ray binaries. The first source is a very-faint millisecond pulsar in the globular cluster M22, for which I conducted a multi-wavelength search for counterparts. The lack of any optical counterpart returned an upper limit on the mass of the companion, allowing for a classification of the system as a so-called black widow binary, i.e., a low-mass X-ray binary with a companion star of mass M ≪ 0.1M⊙. The analysis of the X-ray spectra favoured an intra-binary shock scenario as the mechanism responsible for the X-ray emission. The second source is the well-known high-mass X-ray binary Vela X-1, for which I performed a high-resolution spectroscopy study of a Chandra/HETGS archival data, taken when the line of sight is intersecting the photoionisation wake. Standard plasma diagnostic techniques and simulations with the photoionisation codes CLOUDY and PION (in SPEX) suggested the presence of a multi-component plasma, which is typical for high-mass X-ray binaries with clumpy winds.

The aim of this PhD thesis is to study and characterize the optical and X-ray emission of the afterglows of gamma-ray bursts (GRBs). GRBs are the most powerful sources of electromagnetic radiation in the universe, with an isotropic luminosity that can reach values of $10^{54}$ erg/s. The Swift satellite, launched in November 2004, opened a new era for the study and understanding of the phenomenon of GRBs, thanks to the rapid response of its narrow FOV instruments that allows the accurate localization of most GRBs and the more complete coverage of the GRB evolution. In the first part of my PhD I was involved in a comprehensive statistical analysis of the Swift X-ray light-curves (LCs) of GRBs, carried out in a model-independent way. Our sample is composed of the X-ray LCs of more than 650 GRBs observed by Swift from December 2004 to December 2010. For 437 GRBs the statistics were good enough to allow us to extract a spectrum to convert their count-rate LCs into flux LCs. For GRBs with a known redshift, also rest-frame luminosity LCs in the 0.3-30 keV band were computed. From the fit of these LCs, we obtained the values of the temporal slopes and break times of the continuum of the X-ray emission, since the used fitting procedure automatically discards the positive fluctuations (i.e. flares). Then, we computed the total fluences and energies, those of flares and differentiating between the components of the X-ray LCs. Thanks to this large sample of LCs, we could carry out a homogeneous analysis of GRBs in a common rest frame energy band (0.3-30 keV), investigating the intrinsic time scales and energetics of the different LC phases. In addition, we studied the properties of flares superimposed to the smooth X-ray decay. GRBs are classified as long and short, depending on the duration of the prompt emission (T90>2 s and T90<2 s, respectively); our sample of GRBs allowed us to investigate the possible differencies and similarities between these two classes, for example the nature of long and short GRBs and the emission mechanisms involved. Finally, we examined the possible relation between the X-ray and gamma-ray emission and we found the existence of a universal scaling involving two parameters of the prompt emission and one of the X-ray emission: the isotropic prompt emission energy ($E_{\gamma,iso}$), the peak energy ($E_{pk}$) and the isotropic X-ray energy ($E_{X,iso}$). The main idea of the project presented above is to study all quantities that characterize the X-ray data and to look for a link between prompt and afterglow emission. During this work, we realized that the optical data were very important for our understanding, adding information to investigate the GRB emission mechanisms and to study the environment properties. Therefore, in the second part of my PhD we carried out a systematic analysis of the optical data available in literature, collecting data from all the available sources. From the collected optical data, we determine the shapes of the optical LCs. Then, we modeled the optical/X-ray spectral energy distribution (SED), we studied the SED parameter distributions and we compared the optical and X-ray LC slopes and shapes. For 20% of GRBs the difference between the optical and X-ray slopes is consistent with 0 or 1/4 within uncertainties (we do not consider here the steep decay phase), but in the majority of cases (80%) the optical and X-ray afterglows show significantly different temporal behaviors. Interestingly, we found an indication that the onset of the forward shock in the optical LCs (initial peaks or shallow phases) could be linked to the presence of the X-ray flares. Indeed when there are X-ray flares the optical LC initial peak or plateau end occurs during the steep decay, instead if there are no X-ray flares or if they occur during the plateau, the optical initial peak or plateau end takes place during the X-ray plateau. This could link the prompt emission with the optical emission. The forward shock model cannot explain all the features of the optical (e.g. bumps, late re-brightenings) and X-ray (e.g. flares, plateaus) LCs. However, the synchrotron model is a viable mechanism for GRBs afterglow emission at late times. Further to the intrinsic spectrum of the afterglow, the SED analysis allows to study the properties of the GRB environment, by quantifying the amount of absorption at optical and X-ray wavelengths. The first is due to dust while the latter is mostly due to metals. Our analysis shows that the gas-to-dust ratios of GRBs are larger than the values calculated for the Milky Way, the Large Magellanic Cloud, and the Small Magellanic Cloud assuming solar abundances.<br>Lo scopo di questa tesi è lo studio e la caratterizzazione dell'emissione X e ottica dei gamma-ray burst (GRB). I GRB sono la sorgente più potente di radiazione elettromagnetica dell'universo, la cui luminosità può raggiungere valori di $10^{54}$ erg/s. Il satellite Swift, lanciato nel novembre del 2004, ha aperto una nuova era per lo studio e la comprensione dei GRB, grazie alla rapida risposta dei suoi strumenti che ha permesso localizzare in modo accurato la maggior parte dei GRB e di ottenere una visione più completa della loro evoluzione. Nella prima parte del mio Dottorato sono stata coinvolta nell'analisi statistica delle curve di luce (CL) osservate nella banda energetica corrispondente ai raggi X del telescopio per i raggi X a bordo del satellite Swift. Questo studio non ha assunto alcun modello teorico per spiegare le osservazioni, ma è stato finalizzato alla raccolta di tutte le possibili informazioni osservative. Il nostro campione è composto dalle CL di più di 650 GRB osservati da Swift tra Dicembre 2004 e Dicembre 2010. Per 437 GRB, grazie alla bontà statistica dei dati, è stato possibile estrarre uno spettro per convertire le loro CL da conteggi a flusso. Per i GRB per cui è stato misurato il redshift, sono state calcolate anche le CL in luminosità nella banda energetica 0.3-30 keV nel sistema di riferimento della sorgente, in modo da approssimare la luminosità bolometrica. Dall'interpolazione dei dati delle CL, abbiamo ottenuto i valori delle pendenze temporali e dei break time, cioè dei tempi in cui la CL cambia la sua pendenza, e abbiamo caratterizzato l'andamento temporale dell'emissione duratura in banda X, escludendo le fluttuazioni (flares) che sono probabilmente dipendenti da meccanismi diversi. Per ogni GRB, sono state calcolate le densità di flusso e le energie corrispondenti all'emissione X totale, dei flares e delle diverse componenti della CL. è stata realizzata un'analisi omogenea dei GRB in una banda energetica comune (0.3-30 keV) nel sistema a riposo della sorgente. I GRB sono classificati come lunghi e corti, in base alla durata dell'emissione iniziale, detta prompt emission (T90>2 s e T90<2 s, rispettivamente); il nostro campione di GRB ci ha permesso di studiare le differenze e le somiglianze di queste queste due classi di GRB. Infine abbiamo identificato una nuova relazione tra l'emissione X e gamma trovando una legge universale che coinvolge due parametri che si riferiscono alla prompt emission e uno che si riferisce all'emissione X: l'energia totale della promp emission ($E_{\gamma,iso}$), l'energia di picco dello spettro integrato nel tempo della prompt emission ($E_{pk}$) e l'energia X ($E_{X,iso}$). L'idea principale del progetto appena discusso è lo studio di tutte le quantità che caratterizzano i dati X e la ricerca di un legame tra l'emissione prompt nei raggi gamma e quella nelle altre bande energetiche, X, ottico e radio, detta afterglow. Durante questo lavoro, ci siamo resi conto della necessità di aggiungere le informazioni che provengono dai dati ottici dei GRB, in modo da studiare in modo più dettagliato i meccanismi di emissione dei GRB e le proprietà dell'ambiente che li circonda. Quindi, nella seconda parte del mio Dottorato ho condotto un mio personale progetto di ricerca, analizzando in modo sistematico i dati ottici disponibili in letteratura. Il primo passo è stato quello di interpolare le CL ottiche, in modo da caratterizzare il loro andamento temporale. Poi abbiamo modellato le distribuzioni di energia spettrale ottica e X (SED) e abbiamo studiato le distribuzioni dei parametri ottenuti da questo studio. Infine abbiamo confrontato l'andamento temporale delle CL ottiche. Per il 20% dei GRB la differenza tra la pendenza ottica e X è consistente con i valori attesi dal modello standard per l'afterglow dei GRB, mentre nella maggior parte dei casi le CL ottiche e X mostrano un andamento temporale diverso. Inoltre, abbiamo trovato un'indicazione che l'inizio della fase di afterglow nelle CL ottiche (che corrisponde nelle CL a picchi iniziali o fasi quasi-costanti) potrebbe essere collegato alla presenza dei flare nei raggi X. Quindi, quando ci sono flares X, il picco iniziale o la fine della fase quasi-costante della curva di luce ottica avvengono durante la fase iniziale della CL X, detta steep decay, invece se non ci sono flare X o se avvengono successivamente allo steep decay, il picco iniziale o la fase quasi-costante della CL ottica si manifestano durante la fase quasi-costante della curva di luce X. Questo potrebbe legare l'emissione prompt con l'ottico. In generale, troviamo che il modello del standard per l'afterglow non può spiegare tutte le caratteristiche delle CL ottiche e X. Comunque, l'emissione di sincrotrone può essere un meccanismo plausibile per spiegare l'emissione dell'afterglow a tempi tardi. L'analisi delle SED ci ha permesso di studiare le proprietà dell'ambiente dei GRB, quantificando la quantità di assorbimento alle lunghezze d'onda ottiche e X. Il primo è dovuto alla polvere invece l'ultimo è dovuto principalmente ai metalli. La nostra analisi ha mostrato che il rapporto tra il gas e la polvere per i GRB è maggiore rispetto ai valori calcolati per la Via Lattea, la Grande Nube di Magellano e la Piccola Nube di Magellano, assumendo abbondanze solari.

Improvements in the design, fabrication, and performance of astronomical detectors has ushered in the so-called era of multi messenger astrophysics, in which several different signals (electromagnetic waves, gravitational waves, neutrinos, cosmic rays) are processed to obtain detailed descriptions of their sources. Soft x-ray instrumentation has been developed in the last decades and used on board numerous space missions. This has allowed a deep understanding of several physical phenomena taking place in astrophysical sources of different scales from normal stars to galaxy clusters and huge black holes. On the other hand, imaging and spectral capabilities in the the hard x-rays are still lagging behind with high potentials of discovery area. Modern cryogenic microcalorimeters have two orders of magnitude or more better energy resolution with respect to CCD detectors at the same energy in the whole X-ray band. This significant improvement will permit important progress in high energy astrophysics thanks to the data that will be provided by future missions adopting this detector technology such as the ESA L2 mission Athena, the JAXA/NASA mission XRISM, both under development, or the NASA LYNX mission presently under investigation. The JAXA/NASA mission Hitomi, launched in 2016 and failed before starting normal operation, has already given a hint of the high potential of such detectors. Due to their very high sensitivity, X-ray cryogenic microcalorimeters need to be shielded from out of band radiation by the use of efficient thin filters. These microcalorimeters work by measuring the temperature increase caused by a photon that hits an X-ray absorber. In neutron transmutation doped germanium (NTD Ge) devices the temperature increase in the absorber is measured by a semiconductor thermometer made of germanium doped by the neutron transmutation doping technique. They are characterized by relatively low specific heat and low sensitivity to external magnetic fields. These characteristics make them promising detectors for hard X-ray detectors for space and laboratory applications. Research groups of the the X-ray Astronomy Calibration and Testing (XACT) Laboratory of the Osservatorio Astronomico di Palermo – Istituto Nazionale di Astrofisica (INAF-OAPA), and of the Dipartimento di Fisica e Chimica “Emilio Segrè” (DiFC) of the Università di Palermo have already developed experience related to the design, fabrication and testing of NTD Ge microcalorimeters. Furthermore, the research group has participated for many years in the design and development of filters for x-ray detectors in different space missions. This thesis concerns the development of materials and technologies for high energy microcalorimeters. In particular its aim is to design and fabricate thick bismuth absorbers for NTD germanium microcalorimeter arrays to extend their detection band toward hard X-ray energies. Filters for shielding microcalorimeters from different background radiation arriving on the detectors were also studied. The design and fabrication of thick bismuth absorbers for hard x-rays detection (20 keV ≤ E ≤ 100 keV) is part of an ongoing effort to develop arrays of NTD Ge microcalorimeters by planar technologies for astrophysical applications. One potential application of such detectors is in the high spectral resolution (∆E ~ 50 eV) investigation of the hard X-ray emission from the solar corona, which is the goal of a stratospheric balloon borne experiment concept named MIcrocalorimeters STratospheric ExpeRiment for solar hard X rays (MISTERX) presently under study at INAF-OAPA. The characterization activity of filters for microcalorimeters in also related to the implementation of the European Space Agency high energy mission named Athena (Advanced Telescopes for High Energy Astrophysics). This thesis describes the design, fabrication, and characterization of the bismuth absorbers, as well as the characterization of filters for Athena. Chapter one summarizes the working principles of NTD Ge microcalorimeters and their applications. Chapter 2 describes the design of the bismuth absorber array on suitable substrates. Chapter 3 focuses on the electroplating process for the bismuth layer deposition, with details about the design and fabrication of the microlithographic mask for the array patterning, and about the development of the microlithographic process for the array fabrication on the chosen substrates. The fabrication of 4 x 4 absorber arrays is also described. Chapter 4 reports on the characterization activity of deposited bismuth layers by different techniques; their morphology was investigated by scanning electron microscopy. The electrochemical impedance spectroscopy technique was used to increase grown layer quality. Fabricated arrays were also characterized. Chapter 5 describes the characterization activity for different filter prototype samples developed for Athena. Mechanical robustness, radio frequency attenuation and radiation damage caused by protons were evaluated. Radiation damage effects at different doses were in particular investigated on silicon nitride filters by scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-Vis-IR spectroscopy and x-ray attenuation measurements. Details on both technical detector requirements and different sensor types are given in the Appendix.

Gamma–Ray Bursts (GRBs) are the most distant objects ever detected after the recombination epoch. They consist of a short intense emission episode of gammarays (10 keV–2 MeV) with typical duration between 10−2 and 103 seconds. This is called the “prompt” emission phase. GRBs are classified, according to their observed duration, into short GRBs (lasting less than 2s) and long GRBs (lasting more than 2 s). During the prompt phase GRBs are the brightest objects in the gamma–ray sky. The gamma–ray prompt emission is accompanied by a long lasting emission, called “afterglow”, covering the whole spectral range from the radio to the X– rays. The afterglow emission can be observed up to months after the prompt phase ceased. After the discovery of the GRB afterglow made possible by the Dutch-Italian satellite BeppoSAX, and the confirmation of their cosmological origin, the GRB community reached a general consensus about the nature of these sources which led to the formulation of the so called “standard fireball model”. This model was able, until recently, to account for most of the observational properties of the both the prompt and the afterglow emission. In this scenario, long GRBs are thought to be produced by the core collapse of massive stars. The gamma–ray prompt emission is produced by the “internal shocks” developed by the collisions of different plasma shells ejected by the central engine with different Lorentz factors. The afterglow emission is due to the “external shock” produced by the deceleration of a relativistically expanding fireball by the external medium. The leading radiative mechanism responsible for the prompt and the afterglow emission is synchrotron radiation by electrons accelerated at the internal/external shocks. An important assumption of the standard model is that both the optical and the X–ray afterglows are produced by the same mechanism, taking place in the same region. The launch of the Swift satellite (in November 2004), in synergy with the available network of automatic ground based optical telescopes, signed a remarkable improvement (a kind of “revolution”) of our ”view” of GRB afterglows. Thanks to the fast repointing capabilities of Swift, now X–ray and optical afterglows can be observed starting only few minutes after the prompt GRB emission. Before the launch of Swift, instead, afterglow observations started typically several hours after the burst detection. This new observational window, opened on the early times afterglow emission, unveiled a picture that is much more complex than what had been seen before Swift when the optical and X–ray light curves were usually well described by simple power law decays. The early time light curves observed in the X–rays (and sometimes in the optical), show different phases characterised by different decay indices, chromatic breaks and sudden rebrightenings. Another important finding of Swift is the fact that often the GRB optical light curve does not track the X– ray one. This cannot be explained in the framework of the standard model which assumes that both the X–ray and the optical emission have the same origin and, therefore, should behave similarly. For this reason, in the last few years, several alternative models have been proposed in order to account for the new “afterglow picture” depicted by the Swift observations. Most of these models, however, try to reconcile the observed X–ray and optical light curve complexity through some modifications of the standard afterglow model. Usually, these alternative scenarios assume, as in the standard model, that the optical and X–ray emission are due to the same emission mechanism operating in the same emitting region and therefore suffer of the same main problem of the standard model i.e. they can hardly reproduce the diverse light curves of the optical and X–ray emission of individual GRBs. My thesis is devoted to the study of this issue, i.e. the study of the GRB afterglows to understand the physical mechanisms that produce the observed optical and X–ray emission. The aim of my thesis is to study and to test with the available observations a possible alternative scenario to the standard model that fails to explain the complex behaviour of the X-ray and optical afterglow emission of GRBs. To this aim I studied the intrinsic (i.e. rest frame) afterglow properties simultaneously taking into account the optical and X–ray light curves. This is possible exploiting the rich broad band follow up that is now available for a large number of events. I analysed the optical luminosities of long GRBs finding an unexpected clustering and bimodality of the optical luminosity distributions. I proved that these results are not due to observational selection effects and that the X–ray luminosity are not in agreement with what found in the optical. These results can hardly be explained in the framework of the standard afterglow model. Together with the group I am working with, I analysed the light curve of the optical and X–rays rest frame luminosity of a sample of 33 long GRBs. We modelled the broad band light curve evolution as due to the sum of two separate components, contrary to the usual assumption of a common origin of the optical and X–ray emission. We obtain a good agreement with the observations, accounting for the light curves complexity and diversity. This two component model makes predictions about the broad band spectral energy distribution (SEDs), that I tested analysing the observed SEDs. Through this analysis I confirm that our two component model is consistent with the observed data also form the spectral point of view. This led us to propose a new view of the afterglow emission mechanism following the so called late prompt scenario proposed by Ghisellini et al. 2007. According to our view, the central engine activity lasts for long time (up to months after the trigger) keeping on producing slower shells that are responsible for the emission of optical and X–ray radiation that competes with the standard forward shock emission. This generates the complexity of the observed broad band light curves and explains the diversity between the optical and X–ray temporal evolution. We suggest that the late time activity of the central engine is sustained by the accretion of the material that failed to reach the escape velocity from the exploding progenitor star, and falls back. The presence of this mechanism is strengthened by the similarity between the temporal evolution of the late prompt component, and the expected time profile of the accretion rate of the fall back material.

La Tesi di Dottorato di Ricerca, svolta all’IASF CNR/INAF di Roma sotto la supervisione del dott. Enrico Costa, contiene lo studio delle prestazioni scientifiche dello strumento SuperAGILE. SuperAGILE è il monitor a raggi X di AGILE, missione su satellite dell’ASI composta da due strumenti, sensibili rispettivamente nelle bande di energia 15-40 keV e 30 MeV-50 GeV, il cui lancio è previsto per la fine del 2005. SuperAGILE è uno strumento a maschera codificata, con rivelatore a microstrip di silicio e maschera di tungsteno. Oggetto della mia Tesi di Dottorato è lo studio delle prestazioni scientifiche di SuperAGILE e delle più importanti criticità dello strumento: misura dell’uniformità delle prestazioni del circuito XAA1.2 dell’elettronica di lettura, della sua stabilità termica e della stabilità per variazioni della tensione di alimentazione, studio dell’interazione dei raggi cosmici nel circuito con misure sperimentali e stima del flusso aspettato in orbita, misura delle prestazioni scientifiche del modello da volo di SuperAGILE e, infine, studio dell’effetto della disuniformità di soglia sulle immagini. Le misure dell’uniformità di prestazioni dell’XAA1.2, della stabilità termica (tra –20° C e +40° C) e della stabilità per variazioni della tensione di alimentazione si effettuano con una scheda di acquisizione dedicata e stimolando il circuito per mezzo di un generatore di impulsi di tensione all’interno della scheda. Dalle misure si trova una variazione dei segnali di indirizzo dell’XAA1.2 (fondamentali per ricostruire le immagini delle sorgenti in Cielo) sulla scala dei 10° C. Lo studio dell’effetto dell’interazione dei raggi cosmici nel circuito, non progettato per applicazioni spaziali, riguarda il latch-up (aumento delle correnti di alimentazione che può danneggiare il chip per surriscaldamento) e il SEU (variazione di un bit nella memoria con perdita di programmazione) e l’effetto della dose assorbita sulla linearità e sul consumo. La misure sperimentali sono state effettuate irraggiando l’XAA1.2 con ioni (da 16O a 197Au) all’esperimento dedicato SIRAD dell’acceleratore Tandem nei Laboratori Nazionali INFN di Legnaro presso Padova. Al variare del LET, che misura l’energia per unità di lunghezza rilasciata dalle particelle cariche nel silicio, si misura la sezione d’urto di latch-up e SEU. Ad intervalli regolari si effettuano misure di linearità con l’impulso di calibrazione per studiare l’effetto della dose. Stimando il flusso di ioni in orbita con il codice CREME96 e usando un modello approssimato che tiene conto della spallazione dei protoni, ho trovato che il tasso aspettato di latch-up e SEU in orbita è minore di un evento per tutta la durata di AGILE e che l’effetto della dose assorbita è trascurabile. La Tesi di Dottorato contiene anche la caratterizzazione del modello da volo di SuperAGILE, che consiste nel misurare con l’impulso di carica la linearità e il rumore dell’elettronica di lettura dopo il montaggio degli XAA1.2, dopo la procedura di burn-in (accensione della scheda programmata in configurazione nominale all’interno di un forno a 75° C per 240 ore consecutive) e dopo l’integrazione del rivelatore. Dalle misure si trova che il burn-in non produce variazioni di prestazioni e che, dopo il montaggio del rivelatore, il rumore dell’elettronica di lettura è pari a circa 7.5 keV FWHM mentre l’energia di soglia è circa 19 keV. Il rumore dell’elettronica è stato misurato anche tramite l’acquisizione di sorgenti di raggi X (241Am, 57Co, 109Cd e righe di fluorescenza del Ba) e i valori trovati sono in buon accordo con le misure con l’impulso di carica. La Tesi contiene anche la discussione delle principali problematiche affrontate durante la scrittura di programmi per l’analisi dei dati raccolti in laboratorio. A causa del gran numero di pixel del rivelatore di SuperAGILE, infatti, la linearità e il rumore (sia con impulso di carica che con sorgenti di raggi X) devono essere stimate in modo automatico, senza l’immissione di parametri dall’utente. Infine, ho stimato l’effetto della disuniformità dell’energia di soglia sulle immagini di SuperAGILE generando immagini del fondo sul rivelatore, applicando diversi modelli di disuniformità di soglia e decodificando le immagini del cielo. Mentre l’attuale livello di uniformità di soglia non è sufficiente per poter osservare sorgenti deboli con integrazioni dell’ordine di 106 s, l’equalizzazione fine digitale delle soglie del circuito, tramite un DAC a 3 bit, permette di ottenere una uniformità sufficiente per poter effettuare osservazioni per 106 s.<br>The Ph.D. Thesis, performed at IASF CNR/INAF in Rome under the supervision of dr. Enrico Costa, contains the study of the scientific performances of the SuperAGILE instrument. SuperAGILE is the X-ray monitor of AGILE, satellite-borne mission of ASI whose payload is composed of two instruments, sensitive in the 15-40 keV and 30 MeV-50 GeV energy bands respectively, and whose launch is foreseen in late 2005. SuperAGILE is a coded aperture instrument with silicon microstrip detector and tungsten coded mask. Topic of my Ph.D. Thesis is the study of the SuperAGILE scientific performances and criticalities: measurement of the performances uniformity of the XAA1.2 front-end electronic circuit, of its thermal stability and of its stability toward supply voltage variations, study of the cosmic rays interaction in the front-end circuit with experimental measurements and estimate of the expected flux in orbit, measurements of the scientific performances of the SuperAGILE flight model and finally study of the impact of the threshold non uniformity on the images. The measurements of the performances uniformity of the XAA1.2, of its thermal stability (between –20° C and +40° C) and of the stability toward supply voltage variations are performed using a dedicated acquisition board feeding the chip with a pulse generator contained in the board. From the measurements a variation of the XAA1.2 address signals (used to reconstruct the images of the sources in the Sky) on the 10° C scale is found. The study of the effect of the cosmic rays interaction in the XAA1.2 chip, that is not designed as a radiation hard component for space applications, concerns the latch-up (sudden increase of the supply currents that can damage the chip due to overheating) and the SEU (bit flip in the memory registers with loss of chip configuration) and the effect of the absorbed dose on the linearity and power consumption. The measurements have been performed with ions irradiation (from 16O to 197Au) at the SIRAD facility of the Tandem accelerator in the Laboratori Nazionali INFN in Legnaro near Padova. With different values of LET, a measure of the energy released per unit length by the charged particles in silicon, the latch-up and SEU cross-section values are measured. During the irradiation linearity measurements using the test pulse generator are performed in order to study the total dose effect. Evaluating the ions flux in orbit with the CREME96 code and using an approximated model to take into account the proton spallation, I have found that the expected latch-up and SEU rate in orbit is less than one event during all the AGILE duration and that the total dose effect is negligible. My Ph.D. Thesis contains also the characterization of the SuperAGILE flight model, performed measuring the linearity and the noise of the front-end electronics after the XAA1.2 integration, after the burn-in procedure (by supplying the board in nominal configuration inside an oven at 75° C for 240 hours long) and after the detector integration. From the measurements I have found no performance degradation after the burn-in procedure. After the detector integration the noise in the front-end electronic is about 7.5 keV FWHM while the energy threshold is about 19 keV. The noise in the front-end electronic has been measured also using X-ray sources (241Am, 57Co, 109Cd and Ba fluorescence lines) and the measured values are in good agreement with the test pulse measurements. My Thesis contains also the discussion of the most important topics in the development of data analysis programs. Because of the big number of the SuperAGILE detector pixels, linearity and noise (using both test pulse generator and X-ray sources) need to be estimated automatically, without requiring the user to provide specific parameters. Finally, the Thesis contains an estimate of the threshold non uniformity on SuperAGILE images by means of background detector images generation applying different non uniformity threshold models. By decoding the resulting Sky images I have found that, while the nominal threshold uniformity does not allow to observe faint sources with exposures of order 106 s, the uniformity level obtained with the digital fine threshold equalization (3 bit DAC), allows expose for 106 s long.

This thesis has as an objective the study of very energetic phenomena in the central regions of active galaxies. Intense star formation is triggered by mergers of galaxies, and the funneling of large quantities of gas toward the inner regions is thought to also cause accretion onto the central black hole. Whether the two processes necessarily take place together as a step of galaxy evolution is still debated. Regardless of the origin of their connection, star formation and strong AGN activity often coexist in the nuclei of galaxies. Even after star formation is no longer ongoing, the nuclear starburst in a merger results in the formation of galactic bulges. Therefore, stellar densities in the inner kiloparsecs of a galaxy are typically high. As stars, or star forming regions, are close to the AGN, interaction between the two is expected to be frequent, and it can result in a multitude of phenomena worthy of study. The thesis thus focuses on high energy processes taking place in regions where AGN activity and stellar presence is likely important. In particular, the goal of the thesis is to study the emission produced by these phenomena, in the range of X-rays to gamma-rays, whether to disentangle their individual contributions or to detect emission resulting from their interaction. The thesis intends to provide data to continue the study of the starburst-AGN connection, and new models to explain the production of gamma rays in other galaxies. Part I focuses on the X-ray analysis of a local sample of LIRGs. The Great Observatories All-Sky LIRG survey (GOALS) is a project to study the brightest infrared galaxies at z < 0.1, in all possible wavelengths. In this part we analyze data for 63 LIRGs in the lower luminosity range of GOALS. We provide X-ray images, fluxes, luminosities and radial profiles for 84 individual galaxies, spanning a wide range of activity types and merger stages. We also provide information on AGN presence using X-ray and IR diagnostics, and compare the results to those derived for the brightest GOALS galaxies. The data provided in this part will help shine light into the importance of AGN presence in LIRGs, in the context of galaxy evolution. Parts II and III focus on the interaction of AGN jets with stellar populations. When the jet of an AGN impacts a stellar wind, or a blob of material, a shock is formed in which particles can be accelerated up to relativistic energies. We estimate, for different scenarios, the dynamical evolution of the obstacles in the jet and gamma-ray emission generated by the accelerated particles. Part II focuses on the presence of whole populations of stars with high mass-loss rates interacting with the jet, whether because they are inside it, or because they expel layers of accumulated shocked material at the moment they penetrate it. Part III deals with single obstacles, in particular the remnant of a supernova explosion that takes place within the jet. In both parts II and III we include dynamical simulations that illustrate the process and how it affects the jet. Part IV focuses on the interaction of AGN jets with the environment at large scales, in this case considering them as possible sources of reionization at z=6. We estimate the number of jetted sources at the epoch of reionization using rest-UV detections of AGN. We then correct the observed density of sources for effects of obscuration using X-ray data. Jets at high redshift interact strongly with the CMB, upscattering its photons up to ionizing wavelengths through IC, and possibly ionizing the ambient medium significantly.<br>Esta tesis tiene como objetivo el estudio de fenómenos altamente energéticos en las regiones centrales de las galaxias activas. La formación intensa de estrellas se produce tras una fusión de galaxias, y este mismo fenómeno puede resultar en la acreción de material sobre el agujero negro supermasivo central (AGN). Incluso cuando el episodio de formación de estrellas ha terminado, este ha dado lugar a un bulbo galáctico; por lo que las densidades estelares en las zonas centrales de las galaxias son típicamente elevadas. Por lo tanto, es de esperar que las interacciones entre estrellas y AGN sean un fenómeno frecuente, que puede dar lugar a una gran cantidad de fenómenos interesantes. En particular, la tesis tiene como objetivo estudiar la emisión producida por estos fenómenos, en el rango de rayos X a rayos gamma, ya sea para distinguir las contribuciones individuales de cada uno o para estudiar la emisión generada en su interacción. La tesis pretende proveer datos para continuar el estudio de la relación entre formación estelar y actividad AGN, y nuevos modelos para explicar la producción de rayos gamma en otras galaxias. La primera parte de la tesis es un estudio en rayos X de galaxias lumínicas en el infrarojo. Se analiza una muestra local, proporcionando imágenes, flujos, luminosidades y perfiles radiales en rayos X de 84 galaxias individuales. También se proporciona información sobre la presencia de AGN en ellas, usando datos en rayos X e infrarojo. La parte central de la tesis se centra en el estudio de la interacción entre jets de AGN y estrellas, ya sea porque estas se encuentran dentro de él, porque le inyectan material al penetrarlo, o porque explotan como supernovas en su interior. Se estudia la evolución dinámica de estos fenómenos, y los rayos gamma producidos. La última parte estima la contribución de los jets de AGN a la reionización del universo, puesto que su interacción con los fotones del fondo cósmico de microondas puede resultar en radiación ionizante a través del mecanismo de Compton inverso.

In the last years, thanks to new generation satellites (mostly Chandra and XMM-Newton), an increasing number of high resolution spectral and timing observations of X-ray emitting isolated neutron stars (INS)has become available. Several new classes of X-ray emitting INSs have been discovered, all of them radio silent or with radio properties much at variance with those of Pulsar Radio (PSRs): X-ray Dim Isolated Neutron Stars (XDINSs), Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs), Central Compact Objects (CCOs), Rotating Radio Transient (RRATs, in one case). AXPs and SGRs represent two peculiar groups of INSs, totalling 15 objects (6 SGRs and 9 AXPs plus one candidate). Although much less noumerous than PSRs, these objects have been extensively studied because of their peculiar and extreme properties. SGRs exhibit bursts and flares with intensity between $\sim 10^2$ and $\sim 10^{11}$ times their quiescent X-ray luminosity ($10^{34} - 10^{36} \ {\rm erg/s})$. In the last decade transient phenomena have been observed from few AXPs, with luminosity increases between few 10 and few 100 times the X-ray quiescent values. The many similarities in the timing and spectral properties among SGRs and AXPs led to the suggestion that they could be two different manifestations of the same phenomenon. These objects are believed to host a Magnetar, an ultra-magnetized neutron star with magnetic field $\sim 10^{14}-10^{15}$ G. In the Magnetar model emission from SGRs and AXPs comes to the expanse of the ultra strong magnetic field rather than of the rotational, gravitational or thermal energy. In Magnetars the external magnetic field can acquire a toroidal component as a consequence of the deformation of the star crust induced by the super-strong internal field. As a result the star magnetosphere twists up and the currents required to support the non-potential field may provide a large enough optical depth to resonant cyclotron scattering (RCS). As a consequence a distortion in the thermal spectra emitting by the star surface is expected, because primary photons gain energy in the repeated scattering with the magnetospheric charges. Spectra emerging from a twisted magnetosphere in presence of RCS has been investigated by several authors (Lyutikov and Gavriil, 2006, Fernandez and Thompson, 2007, Nobili Turolla and Zane, 2008) and applied to SGRs and AXPs spectra by Rea et al. 2008 and Zane et al., 2008. The NTZ model 3D montecarlo simulation is the more advanced tool to compute both lightcurves and spectra. Resulting spectra are described in terms of 5 parameters: the twist angle $\Delta \phi$, the electron bulk velocity $\beta$, the surface temperature $T$ and the two geometrical angles$\xi$ and $\chi$($\xi$ is the misalignement between spin and magnetic axis while $\chi$ is the line of sight inclination with respect to the spin axis). In this thesis the application of NTZ model to two Transient AXPs XTE J1810-197 and CXOU J164710.2-455216 is discussed. XTE J1810-197 exhibited an outburst in late 2002 - early 2003, increasing its luminosity by two orders of magnitude. After the outburst onset the source has been monitored by XMM-Newton between September 2003 and September 2007, showing a decrease in flux until the pre-outburst value has been reached. CXOU J164710.2-455216 exhibited an outburst on 21 September 2006, when it was observed at a luminosity 300 times greater than that measured just five days before. The source has been observed by XMM-Newton 6 times after the outburst onset, between September 2006 and August 2009. In this work, adapted from Albano et al., 2010 paper, a comprehensive study of the pulse profile and spectral evolution of the two TAXPs following their outbursts onsets is presented. By confronting timing data with synthetic lightcurves obtained from the twisted magnetosphere model (Nobili, Turolla and Zane, 2008), we were able to estimate how the physical parameters of the source (surface temperature and emitting area, electron energy, twist angle) evolve in time. The fits of the pulse profiles also allowed us to infer the geometry of the two systems, i.e. the angles between the magnetic and rotational axes and the line of sight. Spectral models, obtained with the parameter values derived for the timing analysis, provide acceptable fits to XMM-Newton data.<br>Negli ultimi anni, grazie alla nuova generazione di satelliti (in particolare Chandra e XMM-Newton), è divenuto possibile osservare, con alta risoluzione, un crescente numero di stelle di neutroni isolate (INSs) che emettono raggi X. Molte nuove classi di INSs che emettono raggi X sono state scoperte, nessuna delle quali emette nel radio, o con proprietà nel radio in disaccordo con quelle delle PSRs: X-ray Dim Isolated Neutron Stars (XDINSs), Anomalous X-ray Pulsars (AXPs) e Soft Gamma-ray Repeaters (SGRs), Central Compact Objects (CCOs), Rotating Radio Transient (RRATs, in un caso). AXPs e SGRs sono due gruppi di INSs con caratteristiche peculiari, formati in tutto da 15 oggetti (6 SGRs e 9 AXPs più una candidata). Anche se meno numerose delle PSRs, questi oggetti sono stati ampiamente studiati proprio in virtù delle loro proprietà peculiari ed estreme. Gli SGRs mostrano bursts e flares con intensità tra le $\sim 10^2$ e le $\sim 10^{11}$ volte la loro luminosit\`a quiescente in banda X ($10^{34} - 10^{36} \ {\rm erg/s}$). Nell'ultima decade fenomeni spettrali transienti sono stati osservati in alcune AXPs, con un aumento di luminosità compreso tra alcune decine e alcune centinaia di volte la luminosità di quiescenza. Le molte similarità nelle proprietà spettrali e di timing di SGRs e AXPs portano a suggerire che questi oggetti possano essere nient'altro che differenti manifestazioni dello stesso fenomeno. Si ritiene che queste sorgenti ospitino una Magnetar, una stella di neutroni ultra magnetizzata con campo magnetico pari a $\sim 10^{14}-10^{15}$ G. Nel modello Magnetar l'emissione da SGRs e AXPs proviene dall'espansione del campo magnetico ultra intenso piuttosto che ad energie di tipo rotazionale, gravitazionale o termico. Nelle Magnetars il campo magnetico esterno potrebbe acquisire una componente di tipo toroidale, come conseguenza della deformazione della crosta della stella indotta dall'intensissimo campo magnetico interno. Il risultato netto è un twist della magnetosfera della stella; inoltre le correnti richieste per supportare il campo non potenziale potrebbero dar luogo ad una profondità ottica sufficientemente spessa per il resonant cyclotron scattering (RCS). Di conseguenza ci si aspetta una distorsione negli spettri termici, dato che i fotoni primari guadagnano energia nei ripetuti urti con le particelle cariche presenti nella magnetosfera. Gli spettri all'uscita della magnetosfera twistata in presenza di RCS sono stati studiati da svariati autori (Lyutikov e Gavriil, 2006, Fernandez e Thompson, 2007, Nobili Turolla e Zane, 2008) e questo modello è stato applicato agli spettri di SGRs e AXPs da Rea et al. 2008 and Zane et al., 2008. Il codice montecarlo 3D creato da Nobili, Turolla e Zane (2008) è lo strumento più avanzato per calcolare curve di luce e spettri. Gli spettri così ottenuti possono essere descritti in termini di 5 parametri: l'angolo di twist $\Delta \phi$, la velocità di bulk degli elettroni $\beta$, la temperatura superficiale $T$ e i due angoli geometrici $\chi$ e $\xi$ ($\xi$ è il disassamento tra campo magnetico e asse di rotazione mentre $\chi$ è l'inclinazione della linea di vista rispetto all'asse di rotazione). In questo lavoro, riadattato dall'articolo Albano et al., 2010, viene presentato un ampio studio dei profili pulsati e dell'evoluzione spettrale delle due TAXPs a partire dall'inizio dell'outburst. Confrontando i dati di timing con le curve di luce sintentiche ottenute con il modello di magnetosfera twistata (Nobili, Turolla e Zane, 2008) siamo stati in grado di stimare l'evoluzione temporale del parametri fisici della sorgente (temperatura superficiale e area emittente, energia degli elettroni, angolo di twist). I fit del profilo pulsato ci permettono anche di asserire la eometria del sistema, e cioè l'angolo fra campo magnetico e asse di rotazione e quello tra asse di rotazione e linea di vista. I modelli spettrali, ottenuti dai valori dei parametri derivati dall'analisi di timing, danno dei fit accettabili dei dati di XMM-Newton.

The thesis is devoted to a study of mass transfer, non-explosive hydrogen burning and the effects of the magnetic field in cataclysmic variables (CVs) in the context of their evolution and possible paths to Supernovae type Ia. I focused on the observational properties of CVs hosting massive white dwarfs (WDs) and/or accreting at high rate. My aims were to detect signatures of quiescent hydrogen burning, follow novae as they settle into quiescence and to assess whether the WD magnetic field alters secular evolution and the outcome of the nova explosions. The thesis consists of three chapters. The first one focuses on observations of VY Scl-type nova-like systems, or "anti-dwarf novae'', exploring the idea that they can burn hydrogen during their optical low states. I analysed all the available archival X-ray and UV observations, in both, their high and their low states, and found that the proposed hydrogen burning at high atmospheric temperatures is ruled out. VY Scl-type stars cannot be Supernova type Ia progenitors since they either burn hydrogen but have too low-mass WDs or undergo rare nova explosions, expelling more material than was accreted. In the second chapter I investigate two post-novae with massive WDs, confirming the magnetic nature of one of them, and revealing that the second one is an intermediate polar candidate. I also show that a nova explosion in a magnetic system can leave an imprint on the surface of a WD, detectable in soft X-rays for several years after the explosion. It may be due to residual localized hydrogen burning, but a more likely explanation is a temperature gradient in the WD atmosphere. The last chapter represents monitoring of the old magnetic nova GK Per performed with Swift, Chandra and NuSTAR telescopes during its recent dwarf nova outburst. The main goal was to study the effects of increased mass transfer through the disk in a magnetic system. I measured the WD spin-up rate, localized the emission sites of different spectral components, revealed shrinking of the inner radius of accretion disk and redistribution of the accretion energy as the mass transfer grows.<br>Questa Tesi è dedicata allo studio del trasferimento di massa, del bruciamento non-esplosivo dell'idrogeno e degli effetti del campo magnetico nelle variabili cataclismiche (CVs), nel contesto della loro evoluzione e della possibile relazione con le Supernovae di tipo Ia (SN Ia). Mi sono concentrata sulle proprietà osservative di CV contenenti nane bianche (WD) massicce e/o che accrescono rapidamente materia. I miei obiettivi sono stati: rilevare diagnostiche di bruciamento quiescente dell'idrogeno, seguire novae che ritornano nella fase quiescente e valutare se il campo magnetico di una WD può alterare l'evoluzione secolare e il risultato delle esplosioni di nova. La tesi è composta da tre capitoli. Il primo capitolo riguarda le osservazioni di sistemi nova-like di tipo VY Scl, o "anti-novae-nane'' (anti-dwarf novae), e vi esploro l'idea che questi sistemi brucino idrogeno durante i loro stati di bassa luminosità ottica. Ho analizzato tutte le osservazioni di archivio nelle bande X e UV, durante gli stati di luminosità ottica elevata e bassa, e ho scoperto che, se anche avviene il bruciamento dell'idrogeno, la temperatura atmosferica non è elevata. Le stelle di tipo VY Scl non possono essere le progenitrici delle SN Ia perchè o bruciano idrogeno ma WD sono di piccola massa, o avvengono rare esplosioni di nova, in cui espellono più massa di quella che hanno accresciuto. Nel secondo capitolo ho studiato due post-novae con nane bianche massicce, confermando la natura magnetica di una di esse, e rivelando che la seconda è una candidata "polare intermedia''. Ho dimostrato anche che un'esplosione di nova in un sistema magnetico può lasciare un'impronta sulla superficie di una WD, osservabile ai raggi X per alcuni anni dopo l'esplosione. Questo effetto può essere dovuto a residuo bruciamento localizzato di idrogeno, ma la spiegazione più probabile è un gradiente di temperatura nell'atmosfera della WD. L'ultimo capitolo descrive come abbiamo seguito la vecchia nova magnetica GK Per con i telescopi spaziali Swift, Chandra e NuSTAR durante la recente esplosione di nova-nana. Il principale obiettivo è stato quello di studiare gli effetti di un aumento di trasferimento di massa attraverso il disco in un sistema magnetico. Ho misurato l'accelerazione delle rotazione della WD, ho localizzato i siti di emissione delle diverse componenti spettrali, e rivelato il restringimento del raggio interno del disco di accrescimento e la ridistribuzione dell'energia dovuta all'accrescimento di materia durante il periodo di aumento del trasferimento di massa.

The PhD project is aimed at studying Gamma-Ray Burst (GRB) sources discovered by Swift from both the observational and theoretical point of view. It is basically divided into three parts: the first is dedicated to the development of a software able to automatically reduce Swift X-ray data; the second part contains the analysis and interpretation of three particularly interesting GRBs in the context of different theoretical models: the problems and the failure of most of them is shown. Finally, a third part is dedicated to the development of a temporal analysis alternative to the Fourier Transform: the Temporal Analysis in the Time Domain (TTD). Although optimized for GRB sources, this technique can be widely used to study the time properties of any source. Swift GRB catalogue in the 0.3-10 keV energy band: I present a homogeneous X-ray analysis of the GRBs for which the X-Ray Telescope (XRT) on board Swift did the follow up. The software is designed to automatically reduce and analyze the data. In particular, for each source the catalogue contains: count-rate light-curves in the 0.3-10 keV, 0.3-1 keV, 1-2 keV, 2-3 keV and 3-10 keV energy bands extracted in three different ways; hardness ratio information; time resolved spectral information; flux and luminosity calibrated light-curves in the different energy bands, where the calibration is done using the time resolved spectral information (this is the major improvement with respect to the existing Swift-XRT catalogue by Evans et al., 2009). Although written for GRB data, the software can be run on any source detected by XRT. Observational constraints to the GRB theoretical models: GRB060904B, possible detection of Nickel emission: the detection of an extra component in the GRB060904B X-ray spectra in addition to the standard single power law behavior is reported. This component can be fit with different models but the addition of a spectral line corresponding to Nickel emission provides the best correspondence: this would add a piece of information we still lack to the GRB-Supernova connection. I investigate the physical properties that the surrounding medium must have in order to produce a spectral feature that can explain the detected emission. I analyze and discuss how and if the detected spectral excess fits in different theoretical models developed to explain the nature of line emission during the afterglow phase of Gamma Ray Bursts (GRB) sources. Transmission and reflection models have been considered. The detected feature can be explained in a funnel scenario with typical opening angle theta around 5 degrees, nickel mass around 0.1 solar masses and T=10^6 K. For theta around 20 degrees, assuming the reprocessing material to be in the SuperNova (SN) shell, the detected emission implies a nickel mass around 0.4 solar masses at T=10^7 K and a metallicity around 10 times the solar value. If the giant X-ray flare that dominates the early XRT light-curve is identified as the ionizing source, the SN expansion began 3000 s before the GRB event. GRB090111, extra soft X-ray emission and peculiar re-brightening: I present a detailed study of GRB090111, focusing on its extra soft power-law photon index Gamma>5 at the very steep decay phase emission (power-law index alpha=5.1, steeper than 96% of GRBs detected by Swift) and the following peculiar X-ray re-brightening. The spectral analysis supports the hypothesis of a comoving Band spectrum with the the peak of the nuF_nu spectrum evolving with time to lower values: a period of higher temporal variability in the 1-2 keV light-curve ends when the E_peak evolves outside the energy band. The X-ray re-brightening shows extreme temporal properties when compared to a homogeneous sample of 82 early flares detected by Swift. While an internal origin cannot be excluded, I show these properties to be consistent with the energy injection in refreshed shocks produced by slow shells colliding with the fastest ones from behind, well after the internal shocks that are believed to give rise to the prompt emission have ceased. GRB081028, an unusually late X-ray afterglow re-brightening: Swift captured for the first time a smoothly rising X-ray re-brightening of clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB): GRB081028. This offers the precious opportunity to study for the first time the rising phase which is likely present in all GRBs but is usually hidden by the prompt tail emission. This constitutes the first manifestation of what is later to give rise to the shallow decay phase. Contemporaneous optical observations reveal a rapid evolution of the injection frequency of a fast cooling synchrotron spectrum through the optical band, which disfavours the onset of the forward shock as the outflow decelerates as the origin of the observed re-brightening. I investigate alternative origins and show that the observations are consistent with the off-axis jet predictions. However, the high energy budget required by this interpretation suggests a different physical origin for the prompt and steep decay phases vs. the afterglow emission component. Strong spectral softening takes place from the prompt to the steep decay phase with the peak energy of the nuF_nu spectrum evolving as fast as t^-7. I track the evolution of the spectral peak energy from the gamma-rays to the X-rays and highlight the problems of the commonly assumed high latitude and adiabatic cooling interpretations. An abrupt switch-off of the central engine after the prompt emission is disfavored: the detected spectral evolution requires some forms of persistent central engine activity during the steep decay. Notably, a softening of both the high and low spectral slopes with time is also observed. I discuss the low on-axis radiative efficiency of GRB081028 comparing its properties against a sample of Swift long GRBs with secure E_gamma,iso measurements: the efficiency results are consistent with the different physical origin hypothesis of the prompt and re-brightening components. Time in the Time Domain (TTD) analysis and short term GRB variability. The time variability in the afterglow and prompt light-curve can provide important clues to the nature of the source that powers the GRB emission and to its surrounding. The power spectrum analysis in the time domain is developed starting from the findings of Li 2001: unlike the Fourier transform this is suitable to study the rms variations at different time scales. A complete characterization of the output of the technique is performed. Although optimized to study the GRB signal, the TTD is able to provide the the variability information of any kind of short, non-repetitive, non-stationary signals. The timing analysis of 252 15-150 keV light-curves of Swift-detected events, reveals the existence of three different classes of GRBs. Moreover, when the cosmological time dilation effect is considered, the distribution of the GRB characteristic variability time scales is found to cluster around 0.6-1 s. A showcase for the application of the TTD analysis to the GRB prompt emission is represented by the naked-eye GRB080319B: this burst shows the presence of two characteristic variability time scales. Both undergo a remarkable evolution during the prompt emission and are strongly energy dependent. Different theoretical interpretations are discussed: none of them is able to account for all the observational findings. The TTD analysis provides an unprecedented description of the temporal properties of the prompt emission: a full interpretation of its meaning and a robust connection to the physics and related models will be the next step of the research.

The main goal of my PhD Thesis was to investigate the nature of ULXs using their multiwave-length emission properties and to extend the treatment of the evolution of their binary systems including the effects of super-Eddington accretion. In this way we constrain the masses of the black holes and donor stars in these systems, and their accretion regime. To this end, we developed a code that enables us to constrain the properties of ULXs binaries from their position on the Color-Magnitude Diagram, from their multiwavelength SED and from additional information available on the systems (such as the age of its parent stellar population). A novelty of this present treatment is the inclusion of super-Eddington accretion, with the possibility to produce the output in the HST photometric system; the extension of the parameter space for BH and donor masses with a proper computation of the orbital angular momentum loss during super-critical accretion; the possibility to model the Multiwavelength emission of ULXs considering the effects of a Comptonzing corona covering the innermost regions of the disc.

The main objective of the thesis is to study the properties of the Galactic population of radio-quiet and thermally emitting isolated neutron stars (INSs). This is done by studying further the existing neutron star sample of nearby seven sources, known as the Magnificent Seven (M7), as well as by searching for new candidates and constraining possible populations. During the thesis, we investigated the proper motions of three of the faintest M7 in X-rays with the satellite Chandra. This work allowed us to constrain the neutron star displacement in two cases as well as to accurately determine the high proper motion of a third source, for the first time in X-rays with a significance approaching 10 standard deviations (Motch, Pires, Haberl, & Schwope, 2007, Ap&SS, 308, 217; Motch, Pires, Haberl, Schwope, & Zavlin, 2009, A&A 497, 423). The search of new INS candidates in the serendipitous catalogue of the XMM-Newton Observatory, with more than 120,000 X-ray sources, had as well the aim to constrain the spatial density of thermally emitting sources located beyond the solar vicinity. This work allowed the long awaited discovery of a new thermally emitting INS with properties similar to those of the seven nearby sources discovered by ROSAT (Pires, Motch, Turolla, Treves, & Popov, 2009, A&A 498, 233). Moreover, deep optical observations with SOAR and the ESO-VLT have been obtained during the thesis work in order to optically identify a handful of INS candidates that have been selected among more than 72,000 sources (Pires, Motch, & Janot-Pacheco, 2009, A&A, 504, 185). Finally, population synthesis of Galactic thermally emitting INSs allows constraining the global properties of this population based on the whole sample of XMM-Newton observations. By estimating the density of similar sources at more remote distances in the Milky Way, the final objective is to determine whether the spatial density derived from the group of seven nearby sources is a local anomaly caused by the Suns current location near regions of active stellar formation of the Gould Belt.<br>O objetivo da tese é estudar as propriedades da população Galáctica de estrelas de nêutrons isoladas com emissão térmica em raios X mas silenciosas em rádio. Isto é feito iinvestigando-se a amostra existente de sete fontes próximas, conhecidas como Magnificent Seven (M7), assim como através da procura por novos candidatos e restringindo possíveis cenários e populações. Durante a tese, nós investigamos os movimentos próprios de três das mais fracas fontes em raios X com o satélite Chandra. Este trabalho nos permitiu restringir o deslocamento da estrela de nêutrons em dois casos assim como medir com grande precisão o alto valor de movimento próprio de uma terceira fonte, pela primeira vez em raios X com uma precisão alcançando 10 desvios-padrão (Motch, Pires, Haberl & Schwope, 2007, Ap&SS, 308, 217; Motch, Pires, Haberl, Schwope & Zavlin, 2009, A&A 497, 423). A procura por novos candidatos a estrelas de nêutrons isoladas no catálogo de fontes do satélite XMM-Newton, com mais de 120.000 fontes de raios X, teve igualmente como objetivo restringir a densidade espacial de fontes com emissão térmica situadas além da vizinhança solar. Este trabalho levou à aguardada descoberta de uma nova estrela de nêutrons isolada em processo de resfriamento, a qual exibe propriedades similares às sete fontes descobertas pelo ROSAT (Pires, Motch, Turolla, Treves, & Popov, 2009, A&A 498, 233). Mais ainda, observações óticas profundas com os telescópios SOAR e ESO-VLT foram obtidas durante a tese de maneira a identificar no óptico a amostra mais brilhante de candidatos a estrelas de nêutrons, os quais foram selecionados entre mais de 72.000 fontes (Pires, Motch, & Janot-Pacheco, 2009, A&A, 504, 185). Finalmente, a síntese de população de estrelas de nêutrons isoladas Galácticas permite restringir as propriedades globais da população com base na amostra total de observações em raios X realizadas com o satélite XMM-Newton. Estimando-se a densidade de fontes similares a maiores distâncias na Via Láctea, o objetivo final é determinar se a densidade espacial derivada do grupo de sete estrelas próximas corresponde a uma anomalia local causada pela proximidade do Sol em relação a regiões de ativa formação estelar do Cinturão de Gould.

Contesto scientifico Le stelle T Tauri classiche (CTTSs) sono oggetti stellari giovani poco massivi che accrescono massa dal loro disco circumstellare. Il disco si estende internamente fino al raggio di tron- camento, ovvero dove il campo magnetico e` abbastanza intenso da sollevare il materiale dal piano del disco e da incanalarlo formando delle colonne di accrescimento (Koenigl 1991). Il materiale incanalato precipita sulla stella e impatta sulla superficie stellare. Gli impatti gener- ano shocks caldi. Le CTTSs sono anche caratterizzate da un alto livello di attivita` coronale, come rivelato dalle osservazioni in banda X (e.g. Favata et al. 2005). Questa attivita` coronale e` prodotta principalmente da flares energetici. Obiettivo di questo lavoro In questo lavoro abbiamo investigato il processo di accrescimento di massa nelle CTTSs. Ab- biamo studiato se l’accrescimento dal disco puo ́ essere il risultato di un’ attivita` coronale e ab- biamo analizzato la struttura e la dinamica della colonna di accrescimento nella regione di im- patto. Abbiamo sviluppato modelli numerici che descrivono: un sistema stella disco sotto gli effetti di un’attivita` coronale in prossimita` della superficie del disco; l’impatto di una colonna di accrescimento su una CTTS. Abbiamo investigato se un’intensa attivita` coronale causata da flares che avvengono in prossimita` del disco di accrescimento puo ́ perturbare la stabilita` del disco interno, distruggere la parte interna del disco e possibilmente innescare fenomeni di accrescimento con tassi di ac- crescimento confrontabili con quelli osservati nelle CTTSs (Colombo et al. 2016). A tal fine, abbiamo sviluppato delle simulazioni 3D magnetoidrodinamiche di una protostella magne- tizzata circondata da un disco di accrescimento. Il modello considera gli effetti della gravita` della stella, della viscosita` nel disco, della conduzione termica (considerando pure gli effetti della saturazione del flusso), delle perdite radiative da parte di plasma otticamente sottile e una funzione parametrizzata per descrivere i flares. Abbiamo esplorato casi caratterizzati da una configurazione di campo magnetico stellare costituita da un ottupolo piu` un dipolo e da diverse densita` o differenti livelli di attivita` di flaring. Per quanto riguarda lo studio degli impatti di accrescimento, abbbiamo analizzato gli ef- fetti della radiazione emergente dal plasma riscaldato dallo shock sulla struttura del materiale pre-shock in caduta. A tal fine abbiamo aggiornato un modulo radiativo implementato nel codice PLUTO (Mignone et al. 2007, 2012) che assume il regime termodinamico locale (LTE). Il modulo e` stato generalizzato anche al caso non-LTE (Colombo et al. 2019a). Abbiamo stu- diato se la radiazione emergente dalla regione di shock viene significativamente assorbita dal materiale ancora non scioccato in caduta libera, e se questo assorbimento puo ́ generare un riscaldamento del materiale pre-shock. Abbiamo sviluppato un modello radiativo magne- toidrodinamico che descrive una colonna di accrescimento che impatta sulla superficie di una CTTS (Colombo et al. 2019b). Il modello considera gli effetti della gravita` della stella, della conduzione termica, delle perdite radiative e anche dell’assorbimento della radiazione dal ma- teriale nel regime non-LTE. Risultati Per quanto riguarda gli effetti sulla stabilita` del disco dell’attivita` di flaring, abbiamo osser- vato, come risultato delle simulazioni di una attivita` flaring intensa, la formazione di diverse loops di plasma caldo confinate dal campo magnetico che collegano la superficie della stella al disco. Tutte queste loops costituiscono una calda corona estesa che produce una luminosita` in banda X confrontabile con i valori tipici osservati nelle CTTSs (Colombo et al. 2019c). L’intensa attivita` di flaring vicina al disco puo ́ perturbarne significativamente la stabilita`. I flares generano un’onda di pressione, la quale viaggia attraverso il disco e ne modifica la con- figurazione. In questo modo, delle colonne di accrescimento vengono generate dall’attivita` di flaring, contribuendo cosi al rate di accrescimento sulla stella. I rates di accrescimento derivati dalle simulazioni assumono valori compresi tra 10−10 e 10−9 M⊙yr−1 (Colombo et al. 2019c). Le colonne di accrescimento possono essere perturbate a loro volta dai flares e interagiscono l’una con l’altra, fondendosi per formare colonne piu` grandi. Il risultato di questa dinamica e` che le colonne di accrescimento hanno una struttura molto disomogenea, costituita da blob densi. Questa disomogeneita` potrebbe essere l’origine della variabilita` osservata nelle strutture di accrescimento (Alencar et al. 2018). Il modulo radiativo in non-LTE che e` stato sviluppato per studiare le dinamica e la strut- tura della regione d’impatto nelle CTTSs e` stato validato utilizzando diversi test. In parti- colare, abbiamo realizzato un modello per analizzare la struttura di un semplice shock radia- tivo come descritto da Ensman (1994). Le nostre soluzioni sono in accordo con le soluzioni semi-analitiche (quando disponibili) con una discrepanza massima del 7%. Inoltre, abbiamo provato che un approccio in non-LTE cambia significativamente la struttura e la dinamica della regione di impatto, dando origine ad un precursore radiativo e ad una maggiore esten- sione della regione post-shock rispetto al caso LTE (Colombo et al. 2019a). Il nostro modello radiativo, che descrive la regione di impatto di una colonna di accresci- mento sulla cromosfera stellare, prova che parte della radiazione emessa dal plasma post-shock (≈ 70%) viene assorbita dal materiale freddo pre-shock in caduta sulla stella. L’irraggiamento riscalda il materiale, in caduta sulla stella, ancora non scioccato fino a temperature di 105K. Il materiale caldo forma un precursore termico che emette nella banda UV. I risultati di questo progetto di dottorato potrebbero aiutare nella soluzione di alcune questioni aperte sulle CTTSs. Per esempio, abbiamo provato che un’intensa attivita` di flaring in prossimita` del disco puo ́ perturbarne la stabilita` e generare colonne di accrescimento con una forte strutturazione in densita` come quelle recentemente osservate da Alencar et al. (2018). Inoltre, utilizzando il nostro modello radiativo possiamo spiegare naturalmente l’origine dei complessi spettri in banda UV provenienti dalle regioni di impatto (Ardila et al. 2013) e an- che il fatto che i rates di accrescimento ottenuti dalle osservazioni in banda UV sono sistemati- camente piu` grandi di quelli ottenuti in banda X (Curran et al. 2011). Infatti, il nostro modello radiativo predice la presenza di un precursore termico che emette in banda UV. Questo pre- cursore: 1) Aumenterebbe il flusso UV prodotto dalla regione di impatto senza assumere un piu` alto rate di accrescimento e 2) potrebbe generare un flusso UV prodotto da plasma a ve- locita` di caduta libera sulla stella, quindi con Doppler shifts molto piu` grandi di quelli generati dalla regione post-shock. Questo potrebbe spiegare le componenti ad alto redshift osservate in banda UV (Ardila et al. 2013).<br>Context Classical T Tauri Stars (CTTSs) are young low-mass stellar objects that accrete mass from their circumstellar disks. The disks extend internally up to the truncation radius, where the mag- netic field is strong enough to lift up the material from the disk plane and to funnel the mate- rial forming accretion columns (Koenigl 1991). The funneled plasma falls down onto the star and hits the stellar surface. The impacts generate hot shocks. CTTSs are, also, characterized by high levels of coronal activity, as revealed by X-ray observations (e.g. Favata et al. 2005). This coronal activity is mainly produced by energetic flares. Aims of this work In this work we investigated the mass accretion process in CTTSs. We studied if accretion from the disk to the star might occur as a result of a coronal activity, and we analyzed the structure and the dynamics of the accretion column plasma in the impact regions. We de- veloped numerical models that describe: a star-disk system subject to the effects of a coronal activity in proximity of the disk surface; the impact of an accretion column onto the surface of a CTTS. We investigated if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk, disrupt the inner part of the disk, and possi- bly trigger accretion phenomena with mass accretion rates comparable with those observed in CTTSs (Colombo et al. 2019c). To this end, we modeled a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodynamics simulations. The model takes into account the gravity from the central star, the effects of viscosity in the disk, the thermal con- duction (including the effects of heat flux saturation), the radiative losses from optically thin plasma, and a parameterized heating function to trigger the flares. We explored cases charac- terized by a dipole plus an octupole stellar magnetic field configuration and by either different densities of disk or different levels of flaring activity. As it concerns the study of accretion impacts, we analyzed the effects of radiation emerg- ing from the shock-heated plasma at the base of accretion columns on the structure of the pre- shock downfalling material. To this end, we upgraded a module handling the local thermody- namic equilibrium (LTE) radiation-hydrodynamics (RHD) in the PLUTO code (Mignone et al. 2007, 2012), which we have extended to handle also the non-LTE regime (Colombo et al. 2019a). Then, we investigated if a significant absorption of radiation arising from the shock heated plasma occurs in the unshocked downfalling material, and if it leads to a pre-shock heating of the accreting gas. We developed a radiation hydrodynamics model that describes an accretion column impacting onto the surface of a CTTS (Colombo et al. 2019b). The model takes into account the stellar gravity, the thermal conduction, and the effects of both radia- tive losses and absorption of radiation by matter in non local thermodynamic equilibrium conditions. Results As it concerns the effects of flaring activity on the disk stability, we observed, as a result of the simulated intense flaring activity, the formation of several magnetic loops confining hot plasma that link the star to the disk. All these loops build up a hot extended corona with an X- ray luminosity comparable to typical values observed in CTTSs (Colombo et al. 2019c). The intense flaring activity close to the disk can strongly perturb the disk stability. The flares trig- ger overpressure waves which travel through the disk and modify its configuration. Accretion funnels may be triggered by the flaring activity, thus contributing to the mass accretion rate of the star. Accretion rates derived from the simulations range from 10−10 to 10−9M⊙yr−1 (Colombo et al. 2019c). The accretion columns can be perturbed by the flares and can interact with each other, possibly merging together in larger streams. As a result, the accretion pattern can be rather complex: the streams are highly inhomogeneous, with a complex density struc- ture, and clumped. This inhomogenity may be the origin of the variability observed in the structure of the accretion columns (Alencar et al. 2018). The non-LTE radiation module developed to study the dynamics and structure of the im- pact region of CTTSs has been validated through different tests. In particular, we modeled the structure of a radiative shock, simulating a simple shock case as described by Ensman (1994). The agreement between our solutions and the semi-analytical solutions (when available) is good, with a maximum error of 7%. Moreover, we have proven that a non-LTE approach change significantly the structure and the dynamics of the impact regions, leading to a ra- diative precursor and a greater extension of the post-shock region compared to the LTE case (Colombo et al. 2019a). Our radiative model describing the impact of an accretion column onto the stellar chro- mosphere shows that part of radiation emitted by the post-shock plasma (≈ 70%) is absorbed by the pre-shock accretion column immediately above the slab. The irradiation heats the downfalling unshocked material up to ≈ 105K. This hot material forms a precursor region that emits in the UV band. The results of this PhD project may address some open questions regarding CTTSs. For instance, we proved that an intense flaring activity in proximity of the disk may perturb its stability and may generate accretion columns highly structured in density and characterized by clumps as recently observed, for example, by Alencar et al. (2018). Moreover, with our radiation model we may naturally explain the origin of the complex UV spectra arising from impact regions (Ardila et al. 2013) and the fact that accretion rates derived from UV observations are systematically higher than rates inferred from X-ray ob- servations (Curran et al. 2011). In fact, our model predicts the presence of a precursor region emitting in the UV. This region: 1) would increase the UV flux arising from the impact with- out assuming higher accretion rates and 2) may generate an UV flux produced by plasma at free fall velocity, thus with Doppler shifts stronger than those generated by the post-shock plasma. This may explain the high redshifts and broadening observed in emission lines of UV spectra (Ardila et al. 2013).

I Blazar sono una delle più enigmatiche classi tra i Nuclei Galattici Attivi (AGN). Secondo lo schema unificato degli AGN queste sorgenti sono interpretate come radio-sorgenti con un getto relativistico che è allineato lungo la linea di vista. I Blazar si dividono principalmente in due classi: la prima, costituita dagli oggetti di tipo BL Lac, con spettri in banda visibile privi di righe, e la seconda composta dai quasar con spettro radio piatto (FSRQ) che invece presentano righe di emissione. L’emissione dei Blazar si estende dalla banda radio fino alla banda dei raggi gamma di più alta energia (TeV) e la loro distribuzione spettrale di energia (SED) presenta due picchi: il primo posizionato tipicamente tra la bnada infrarossa e quella dei raggi X, ed il secondo dai raggi γ di basse energie fino alla banda del TeV. Questa distribuzione spettrale è solitamente interpretata in termini di modelli di emissione Synchrotron Self-Compton, secondo cui i fotoni di sincrotrone, corrispondenti alla prima componente, sono emessi da una popolazione di elettroni accelerata nei getti relativistici, e producono la seconda componente attraverso effetto Compton inverso con gli stessi elettroni. Un possibile criterio per classificare gli oggetti di tipo BL Lac è utilizzando la posizione del prima componente della SED e si indicano come HBL le sorgenti che presentano il picco della prima componente nella banda dal UV ai raggi X mrentre LBL quelle in cui questo cade tra l’infrarosso e la banda visibile. Per quanto concerne i FSRQ le loro SED possono essere anche descritte da modelli External Compton, in cui la seconda componente è prodotta per effetto Compton inverso degli elettroni relativistici con fotoni di radiazione esterni al getto. L’obiettivo della Tesi è quello di studiare l’emissione di alta energia (dalla banda dei raggi X fino al TeV) di tutte gli oggetti di tipo BL Lac osservati al TeV fino ad oggi. Mi sono interessato principalmente di investigare i meccanismi di accelerazione ed i processi di emissione nei getti relativistici dei BL Lacs. In particolare, in questo lavoro mi sono occupato dell’evoluzione spettrale delle loro SED nelle bande di energia considerate. A tal fine ho ridotto ed analizzato gli spettri X di 11 anni di osservazioni di BL Lac negli archivi di BeppoSAX, XMM-Newton e Swift. Ho sviluppato alcuni codici numerici per riprodurre le SED dei Blazar. In particolare, ho costruito un codice SSC in grado di calcolare l’emissione per effetto Compton inverso anche al secondo ordine, ed un codice che riproducesse il meccanismo di External Compton dove i fotoni seme sono prodotti da un disco di accrescimento. Tutti i codici sono inoltre in grado di lavorare con diverse distribuzioni in energia degli elettroni. Tra queste distribuzioni la più utilizzata è quella che proviene da meccanismi di accelerazione di tipo statistico/stocastico ed ha una forma log-parabolica. Ogni codice ha inoltre una parte di statistica che permette di determinare i parametri delle SED. Oltre ai codici numerici ho anche sviluppato alcuni calcoli analitici utili per tenere conto del fenomeno della Catastrofe Compton nell’emissione per effetto Compton inverso. Sono state individuate per i BL Lac osservati al TeV alcune correlazioni e trend tra i parametri spettrali dell’emissione X, interpretate in termini di emissione di sincrotrone e meccanismi di accelerazione statistici e stocastici. Un’analisi dettagliata è stata fatta nel caso della Mrk 421, il cui comportamento nel paino dei parametri spettrali non sembra essere dominato da variazioni del fattore Doppler come richiesto dai modelli di “internal shock” per i getti dei BL Lac. E' stata fatta anche una ricerca di questi trend in tutti negli spettri X degli altri HBL osservati al TeV ed almeno in quattro di loro sono stati confermati comportamenti nello spazio dei parametri analoghi al caso della Mrk 421 . Alcune SED degli HBL osservati al TeV sono state confrontate con i calcoli numerici del codice SSC per derivare i parametri fisici di questa classe di sorgenti. Questi parametri sono stati poi utilizzati per descivere le SED di un insieme di HBL per verificarne la possibilità di osservarli al TeV. Infine, i calcoli numerici sono stati anche applicati con successo per descrivere la SED di BL Lacertare, il primo LBL osservato al TeV. Il lavoro sviluppato in questa tesi è quasi interamente dedicato allo studio dei modelli SSC per descrivere le SED degli HBL, con l’ eccezione di BL Lacertae. Una simile analisi potrebbe essere applicata agli oggetti di tipo LBL quando le nuove osservazioni simultanee di entrambe le componenti, sincrotrone ed Compton inverso, saranno disponibili. Questo avverrà dal prossimo anno quando i nuovi satelliti AGILE, GLAS T e Planck forniranno un grande archivio di dati riguardanti l’emissione degli LBL. Il primo periodo di attività di AGILE, durante il quale alcuni LBL ed alcuni FSRQ sono già stati osservati conferma l’importanza di queste osservazioni nei raggi γ per la comprensione dei Blazars.<br>Blazars are a most enigmatic class of active galactic nuclei (AGNs). In the unification scenario of active galactic nuclei (AGNs) Blazars fit in as radio-loud sources with a relativistic jet that points toward us. They are divided in two main classes: the first, less luminous, constitute by the BL Lac objects, with featurelss optical spectra, and the other one composed by the Flat Spectrum Radio Quasars (FSRQs) in which, tipically, there are prominent spectral lines. Blazar emissions extends from radio to TeV energies and their spectral energy distribution (SED) is double bump: the first component tipically peaks from IR to X-ray band, and the second one in the gamma-rays up to TeV energies. Usually, BL Lac SEDs are described in terms of Synchrotron Self-Compton models in which synchrotron photons, emitted by a population of electrons accelerated in the relativistic jets, produce the second component via inverse Compton (IC) scattering by the same electrons. A possible classification criterium for BL Lacs is in terms of the SED peak energy position of the first component: high-frequency peaked BL Lacs (HBLs) comprise sources in which the synchrotron peak is between the UV band and X-rays; low-frequency peaked BL Lacs (LBLs), when the first bump appears in the IR-optical range. FSRQ SEDs tipically require other spectral components to be described, as for example soft seed photons produced in external regions to their jets; so, these emission models are generally named: External Compton radiation. The aim of this thesis is to investigate the high energy emission (from X-rays to TeV frequencies) of all BL Lacs detected at TeV energies, up to now. I am mainly interested in how the synchrotron emission and inverse Compton radiation work in BL Lac jets. My goal is to interpret the spectral evolution of their SEDs in the considered energy ranges. To reach my aim I have reduced and analysed the X-ray spectra of eleven years of archival and new BL Lac observations performed with BeppoSAX, XMM-Newton and Swift. I have developed several numerical codes to reproduce the SEDs of these sources. In detail, I have developed a Synchrotron Self Compton (SSC) code able to reproduce the first and the second order inverse Compton emission, and the External Compton (EC) code that evaluates the inverse Compton scattering between the electron jet population and external radiation field from the accretion disk. Several electrons distribution have been used, in particular, those produced by statistical/stochastical acceleration processes (Fokker-Planck equation), leading to log-parabolic electron distributions. Each code has also a statistical section to determine spectral parameters of the BL Lac SEDs. Besides this numerical codes some analytical calculations have been developed to take into account of the so called Compton Catastrophe in the inverse Compton emission. Some correlations and trends between spectral parameters were found in X-ray observations of the sample of TeV BL Lacs, and they have been interpreted in terms of synchrotron emission and statistical/stochastical acceleration. A detailed statistical analysis was performed for Mrk 421, whose behavior, in the plane of spectral parameters, appears not to reflect beaming variations, as required by “internal shock” models of BL Lac jets. An extensive search of the similar trends was performed on eleven years of X-ray observations with BeppoSAX, XMM-Newton and Swift. I discovered similar trends in HBLs detected at TeV energies at least four sources. Numerical simulations provide a good description of the SEDs of HBLs in my sample. These calculations provide constraints on input parameters for the numerical simulations on HBLs. The constraints derived have been used to describe a sample of HBLs candidate as TeV emitters. Finally, I have also simulated the SED of BL Lacertae, the first LBL observed at TeV energies, discussing among SSC codes and EC simulations. The work developed in this thesis is quasi entirely dedicated to the study of SSC models to describe HBL SEDs, with the only exception of BL Lacertae. A similar analysis could be applied to other LBL objects when the new simultaneous observations of both the synchrotron and the inverse Compton will be avaiable. These will occur in the next year when the new satellites AGILE, GLAST and Planck will provide a large number of data on rich LBL samples. The first period of activity of AGILE in which some LBLs and FSRQs have already been detected confirmed the relevance of these γ-ray observations for the understanding of the BLazars phenomenon.