Rozprawy doktorskie na temat „Gamma Ray Burst (GRB)”

We present the second multi-frequency radio detection of a reverse shock in a gamma-ray burst. By combining our extensive radio observations of the Fermi-Large Area Telescope gamma-ray burst 160509A at z - 1.17 up to 20 days after the burst with Swift X-ray observations and ground-based optical and near-infrared data, we show that the afterglow emission comprises distinct reverse shock and forward shock contributions: the reverse shock emission dominates in the radio band at. less than or similar to 10 days, while the forward shock emission dominates in the X-ray, optical, and near-infrared bands. Through multi-wavelength modeling, we determine a circumburst density of n(0) approximate to 10(-3) cm(-3), supporting our previous suggestion that a low- density circumburst environment is conducive to the production of long-lasting reverse shock radiation in the radio band. We infer the presence of a large excess X-ray absorption column, N-H approximate to 1.5. x 10(22) cm(-2), and a high rest-frame optical extinction, A(V) approximate to 3.4 mag. We identify a jet break in the X-ray light curve at t(jet) approximate to 6 days, and thus derive a jet opening angle of theta(jet) approximate to 4 degrees, yielding a beaming-corrected kinetic energy and radiated gamma-ray energy of E-K approximate to 4 x 10(50) erg and E-gamma approximate to 1.3 x 10(51) erg ( 1-10(4) keV, rest frame), respectively. Consistency arguments connecting the forward shocks and reverse shocks suggest a deceleration time of t(dec) approximate to 460 s approximate to T-90, a Lorentz factor of Gamma( t(dec)) approximate to 330, and a reverse-shock-to-forward-shock fractional magnetic energy density ratio of R-B equivalent to is an element of(B, RS)/is an element of(B, FS) approximate to 8. Our study highlights the power of rapid-response radio observations in the study of the properties and dynamics of gamma-ray burst ejecta.

We present multi-wavelength observations and modeling of the exceptionally bright long gamma-ray burst GRB 160625B. The optical and X-ray data are well fit by synchrotron emission from a collimated blastwave with an opening angle of theta(j) approximate to 3 degrees.6 and kinetic energy of E-K approximate to 2 x 10(51) erg, propagating into a low-density (n approximate to 5 x 10(-5) cm(-3)) medium with a uniform profile. The forward shock is sub-dominant in the radio band; instead, the radio emission is dominated by two additional components. The first component is consistent with emission from a reverse shock, indicating an initial Lorentz factor of Gamma(0) greater than or similar to 100 and an ejecta magnetization of R-B approximate to 1-100. The second component exhibits peculiar spectral and temporal evolution and is most likely the result of scattering of the radio emission by the turbulent Milky Way interstellar medium (ISM). Such scattering is expected in any sufficiently compact extragalactic source and has been seen in GRBs before, but the large amplitude and long duration of the variability seen here are qualitatively more similar to extreme scattering events previously observed in quasars, rather than normal interstellar scintillation effects. High-cadence, broadband radio observations of future GRBs are needed to fully characterize such effects, which can sensitively probe the properties of the ISM and must be taken into account before variability intrinsic to the GRB can be interpreted correctly.

O presente trabalho tem o objetivo principal de compreender os possíveis efeitos da radiação energética de um evento de Gamma-Ray Burst (GRB) sobre o meio interestelar no entorno de seu local de geração e em planetas possivelmente iluminados. Gamma-Ray Bursts foram detectados pela primeira vez nos anos 60 e rapidamente atraíram a atenção da comunidade astrofísica, uma vez que as energias emitidas apenas em poderiam alcançar 1054erg, o equivalente a massa de repouso do Sol. Não se conhecia nenhum mecanismo tão eficiente para extrair energia gravitacional para produzir tal evento. Mais tarde, a possibilidade da emissão ser colimada abaixou a energia em para 5x1050erg, mas o mecanismo central de geração ainda não foi completamente desvendado, havendo muito espaço para alternativas exóticas. Estudamos os efeitos de um GRB sobre o meio interestelar, em uma tentativa de distinguir os remanescentes do GRB do gerado por múltiplas supernovas. Usamos argumentos energéticos e sobre a possibilidade de alterações químicas e isotópicas devido a reações fotonucleares. Também trabalhamos com as implicações biológicas da iluminação de planetas por um GRB, concluindo que os efeitos de tais eventos podem afetar seriamente a biosfera de um planeta mesmo a distâncias de ~10kpc.
The present work has the main goal of understanding the possible effects of the hard gamma radiation produced during a Gamma-Ray Burst (GRB) event both on the interstellar medium surrounding the source of the burst and on planets possibly illuminated. Gamma-Ray Bursts were first detected on the 60s and quickly have attracted the attention of the astrophysical community, since the energies emitted just in could reach 1054erg, the rest mass of the Sun. No mechanism was known to be so efficient in extracting gravitational energy to produce such emission. Later on, the possibility of the emission being collimated has lowered the energy of the to 5x1051erg, but the central engine has not yet been completely understood, and there is still ample room for exotic alternatives. We have studied the effects of GRB on the ISM, in an attempt to distinguish the candidates of GRB remnants from those generated by multiple supernovae. We have used both energetic arguments and the possibility of chemical alterations due to photonuclear reactions. We have also worked on the biological implications of the illumination of planets by a GRB, concluding that the effects of such event could seriously harm the biosphere of a planet even at distances of ~10kpc.

We present the discovery of the X-ray and optical afterglows of the short-duration GRB 150101B, pinpointing the event to an early-type host galaxy at z = 0.1343 +/- 0.0030. This makes GRB 150101B the most nearby short gamma-ray burst (GRB) with an early-type host galaxy discovered to date. Fitting the spectral energy distribution of the host galaxy results in an inferred stellar mass of approximate to 7 x 10(10) M-circle dot, stellar population age of approximate to 2-2.5 Gyr, and star formation rate of less than or similar to 0.4M(circle dot) yr(-1). The host of GRB 150101B is one of the largest and most luminous short GRB host galaxies, with a B-band luminosity of approximate to 4.3L(*) and half-light radius of approximate to 8 kpc. GRB 150101B is located at a projected distance of 7.35 +/- 0.07. kpc from its host center and lies on a faint region of its host rest-frame optical light. Its location, combined with the lack of associated supernova, is consistent with an NS-NS/NS-BH merger progenitor. From modeling the evolution of the broadband afterglow, we calculate isotropic-equivalent gamma-ray and kinetic energies of approximate to 1.3 x 10(49) erg and approximate to(6-14) x 10(51) erg, respectively, a circumburst density of approximate to(0.8-4) x 10(-5) cm(-3), and a jet opening angle of greater than or similar to 9 degrees. Using observations extending to approximate to 30 days, we place upper limits of less than or similar to(2-4) x 10(41) erg s(-1) on associated kilonova emission. We compare searches following previous short GRBs to existing kilonova models and demonstrate the difficulty of performing effective kilonova searches from cosmological short GRBs using current ground-based facilities. We show that at the Advanced LIGO/VIRGO horizon distance of 200 Mpc, searches reaching depths of approximate to 23-24 AB. mag are necessary to probe a meaningful range of kilonova models.

The Radio Ice Cherenkov Experiment(RICE) located at the South Pole, is designed to detect the coherent broad-band radio Cherenkov radiation emitted when a high energy (10¹⁵ to 10¹⁸ eV) neutrino interacts with a nucleon in the ice. Observations have identified that Gamma Ray Bursts(GRBs) are possible sites for high energy neutrino production. We consider here GRBs which occurred in the years 2001 to 2005 inclusive during the operational times of RICE. Using GRB photon spectral data, we calculate the neutrino spectra predicted for these GRBs and the subsequent event number expected in RICE. We re-analyze RICE data in small time windows surrounding the GRB burst start times using a refined method involving by eye analysis of this reduced data set and find no neutrino events in the data set. Using the effective volume of RICE appropriate for each GRB we calculate neutrino flux limits for the GRBs. Although the flux limits are several orders of magnitude weaker than the expected flux, the RICE GRB neutrino limits are the only limits in the PeV to EeV energy range.

With energy outputs >~10^51 erg in 0.1-1000 seconds, Gamma-ray Bursts (GRBs) are the most powerful events yet observed in the Universe. As such they are potential probes of the very early Universe, back to the era of re-ionisation and the first stars, but at the same time they have been observed to span a wide range in luminosity and redshift from the relatively local Universe (z~0.0085) out to z~6.29. GRBs divide into two classes based primarily on their duration as measured by T90 (the time taken to observe 90% of the total burst fluence). Long bursts (L-GRBs) have T90>~2 seconds, and shorts (S-GRBs) T90<~2 seconds. Though much has been learned regarding long duration GRBs since the first afterglow discovery in 1997 (including their likely association with massive core collapse supernovae), much remains unknown regarding short duration GRBs. In this work, after a brief historical introduction and review, we present analyses of the angular cross-correlation on the sky of short GRBs from the BATSE catalogue with galaxies in the local Universe sampled from the PSCz Redshift Survey and the Third Reference Catalogue of Bright Galaxies (RC3). In particular we show that 20%+/-8% (1 sigma) of all BATSE short duration bursts (localised to 10 degrees or better) show correlation with galaxy samples (morphological T-type<=4) within ~112 Mpc. Our statistics thus provide evidence that a substantial fraction of BATSE short GRBs show a tendency to be associated with large scale structure on the sky traced by a variety of galaxy types. Short GRBs are believed to be produced in the final merger of compact object (neutron star-neutron star or neutron star-black hole) binaries, though other possible progenitors are known to exist. The short initial spike of a giant flare from a Soft Gamma Repeater (SGR) such as the December 27th 2004 event from SGR1806-20 would have been detectable by BATSE as a short GRB if it occurred in a galaxy within ~30-50 Mpc (assuming a distance to SGR1806-20 of 15 kpc). Using the observed luminosities and rates of Galactic SGR giant flares, as well as theoretical predictions for the rate of binary mergers, we investigate the ability of plausible Luminosity Functions (LF), singly and in combination, to reproduce our observed correlations and a cosmological S-GRB population. We find the correlations are best explained by a separate population of lower luminosity S-GRBs, with properties consistent with them being due to giant flares from extra-galactic SGRs. Overall predicted number counts are a good fit to the observed BATSE number counts, and furthermore, the wider redshift distribution is consistent with the early Swift S-GRB redshift distribution. The three closest GRBs which have been observed to date were all long duration bursts, and we have therefore also searched for cross-correlation signals between the BATSE long GRBs and local galaxies. The three nearby bursts shared several similar properties such as being under-luminous, spectrally soft and of low variability. We have therefore also investigated a subset of L-GRBs with light curve properties similar to these known nearby bursts. The whole sample is found to exhibit a correlation level consistent with zero (1 sigma upper limit=10%, equivalent to 144 bursts) out to a radius of ~155 Mpc, but a spectrally soft, low observed fluence and low variability subset shows a correlation level of 28%+/-16% (=50+/-28 bursts) within 155 Mpc. These results are consistent with low-luminosity, low-variability bursts being a separate sub-class of L-GRBs which may be much more prevalent in the local Universe than their high-luminosity, cosmologically distant counterparts. To investigate this further, we once again examined plausible luminosity functions for single and dual high and low luminosity populations, based on observed intrinsic rates from the literature. The local population was once again found only to be produced to a sufficient level (while maintaining consistency with the observed overall number counts) by a separate low luminosity population with intrinsic rates several hundred times greater than their cosmological counterparts. Constraining the models via the Swift overall redshift distribution instead of threshold-adjusted BATSE number counts showed that the dual LF models were able to produce excellent fits to the entire redshift distribution while adequately reproducing a local population. Finally, suggestions are made as to the direction future work may follow in order to build on these initial investigations, as well as to how observations with future missions and detectors such as Fermi (formerly GLAST), Advanced LIGO and LOFAR may shed further light on nearby GRBs.

My work of thesis focuses on two protagonists of the transient sky, gamma-ray bursts (GRBs) and fast radio bursts (FRBs), tackling both the open issues behind their emission and the technological challenges connected with their observation. In the context of an in-depth understanding of GRBs I studied the popular ”Ep,i–Eiso” correlation. It is fundamental to shed light on the peculiar behaviour of a few events, that appear to be important outliers of the Ep,i–Eiso correlation. In the first part of thesis, I investigate if the locations in the Ep,i/Eiso plane of these bursts may be due to the fact that a substantial fraction of their energy was released in the soft X-ray band, and consequently missed by the instruments that detected. My result suggests that if the two bursts would have been observed by Swift and by eXTP, they may have matched the Ep,i–Eiso relation. This provides strong support to the idea that instrumental biases can make some events in the lower-left corner of the Ep,i–Eiso plane appearing as outliers. GRB prompt emission mechanism is still an open issue, despite the tremendous progress made in the last decades in the comprehension of the GRB phenomenon. It has been realised that only a very broadband analysis (from soft X–rays to MeV) provides a stringent test to the proposed models in the literature. I tackled the problem performing a systematic, broad-band, temporally-resolved spectral analysis of a number of GRBs to test three out of the most popular models: the empirical Band function, and more physically grounded models like the synchrotron (in the form of a double broken power–law), and the Comptonisation model “grbcomp”. I successfully modelled about 3/4 of the entire sample even if, interestingly, roughly 20% spectra are truly problematic for any model. Adding X–ray data turnedout essential (i) to assess the frequent presence of extra components, like in the case of the Band function; (ii) to validate physical models like synchrotron and grbcomp, at the same time, emphasising their limits. In the era of multi-messenger astronomy, the exploration of the early emission from transients is a key task for understanding the encoded physics, while current generation networks of fully-robotic telescopes provide new opportunities in terms of fast followup and sky cover-age. In this context, I designed a robotic pipeline for robotic optical followup of gamma-ray bursts with the Las Cumbr ́es Observatory network aimed at automatically submitting a requestfor observations within 3 minutes from the discovery alert. Via Telegram the pipeline keeps theusers informed, allowing them to take control upon request. The last part of my thesis focuses on FRBs, millisecond-long bursts uniquely detected atradio frequencies, with the only possible exception of FRB 131104, for which aγ–ray transient positionally and temporally consistent was claimed. The aim of my work was testing the systematic presence of an associated transient high-energy counterpart throughout a sample of the FRB population. My result excludes about 94% of Fermi/GBM detected long gamma–ray bursts and about 96% of Fermi/GBM detected short gamma–ray bursts. My result excludes a γ–ray counterpartas fluent as the one possibly associated with FRB 131104 to be a common feature of FRBs.

Gamma-ray bursts (GRBs) are short, intense flashes of 0.1-1 MeV electromagnetic radiation that are routinely observed by Earth orbiting satellites. The sources of GRBs are known to be extragalacitic and located at cosmological distances. Due to the extremely high isotropic equivalent energies of GRBs, which are on the order of Eiso~1054 erg, the gamma-ray emission is believed to be collimated, making them observable only when they are directed towards Earth. The favored progenitor models of GRBs are also believed to emit gravitational waves that would be observable by the current generation of ground-based interferometric gravitational wave detectors. The LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo instruments operated near design sensitivity and collected more than a year of triple coincident data during the S5/VSR1 science run, which spanned the two year interval between November 2005 and October 2007. During this time, GRB detections were being made by the NASA/Goddard Swift Burst Alert Telescope at a rate of approximately 0.3 per day, producing a collection of triggers that has since been used in a coincident GRB-GW burst search with data from the LIGO-Virgo interferometer network. This dissertation describes the search for gravitational waves using the times and locations of 123 below-threshold potential GRB triggers from Swift over the same time period. Although most of the below-threshold triggers are likely false alarms, there is reason to believe that some are the result of actual faintly-observed GRB events. Recent GRB observations indicate that the local rate of low-luminosity GRBs is much higher than previously believed. This result, combined with the possibility of discovering a rare nearby GRB event accompanied by gravitational waves, is what motivates this search. The analysis results indicate no evidence for gravitational waves associated with any of the below-threshold triggers. A median distance lower limit of ~16 Mpc was derived for a typical neutron star-black hole coalescence progenitor assumption.

La natura dell'emissione prompt dei GRBs è ancora incerta, impedendoci di studiare le loro proprietà fisiche strettamente connesse a fondamentali questioni aperte come la composizione del getto e la dissipazione dell'energia. Il tipico spettro osservato di emissione prompt è modellato da due leggi di potenza con pendenze α e β collegate in modo uniforme a un picco di energia Epeak. Il raffreddamento rapido di elettroni per emissione di sincrotrone è il processo radiativo più naturale, data la forma non termica dello spettro osservato e i forti campi magnetici previsti nella regione emittente. Tuttavia, gli spettri osservati di migliaia di GRBs sono più ripidi delle previsioni di sincrotrone. Oganesyan et al.2017 ha recentemente scoperto nei raggi X la presenza di un ulteriore break spettrale, identificato come la frequenza di raffreddamento del sincrotrone. Motivata da questo risultato, ho cercato la presenza di un break ad energie più alte, utilizzando i dati Fermi/GBM. L'analisi dei 10 GRBs più lunghi e luminosi ha rivelato nel ~70% degli spettri la presenza di una energia Ebreak, situata tra ~20 e 500 keV. Le pendenze spettrali al di sotto e al di sopra del break sono consistenti con i valori previsti per l'emissione di sincrotrone in regime di raffreddamento rapido (-2/3 e -3/2, rispettivamente). Le simulazioni spettrali suggeriscono che la separazione di Ebreak da Epeak e la statistica dello spettro ostacolano l'identificazione del break, il che potrebbe spiegare perché questa energia fondamentale non è stata identificata prima. La consistenza con l'emissione di sincrotrone è stata testata anche adattando un modello di sincrotrone allo spettro di GRB 180720B, confermando i risultati ottenuti con la funzione empirica. Inoltre, ho studiato, per la prima volta, la presenza del break in 10 GRB corti: contrariamente a quelli lunghi, i GRB corti non mostrano il break, ma la pendenza spettrale a bassa energia è consistente con -2/3. Questi risultati implicano dei parametri fisici che sfidano il modello standard dei GRBs. Il rapporto piccolo delle frequenze di sincrotrone (vicino all'unità per i GRB corti) implica che le particelle che emettono non si raffreddano completamente. Ciò richiede un basso campo magnetico (B'≲10 G) che implica una regione di emissione distante (R~1016 cm), in contrasto con la tipica variabilità di ~ms dei GRB. Una possibile soluzione può venire dallo scenario protone-sincrotrone: grazie al tempo di raffreddamento più lungo dei protoni, questo scenario può spiegare lo spettro osservato assumendo valori standard sia per il campo magnetico (B'~106 G) che per la distanza (R~1013 cm). Inoltre, ho ampliato il mio studio dello spettro di emissione prompt includendo la sua caratterizzazione a energie più elevate. L'aggiunta dei dati LAT nell'analisi spettrale ha rivelato in 10 burst su 22 la presenza di un cutoff ad alte energie (~100 MeV), che, interpretati come dovuti all'opacità di produzione di coppie, forniscono stime del fattore di Lorentz Γ del getto nell'intervallo 100-400. L'estensione alle alte energie mi ha permesso anche di studiare la pendenza β dello spettro, che è un parametro chiave per vincolare le corrispondenti pendenze p della distribuzione energetica delle particelle accelerate. Assumendo emissione di sincrotrone, ho trovato un'ampia distribuzione di p, centrata intorno a p=2,86 (con una coda fino a p~5-7). Date le incertezze teoriche sulla distribuzione dell'energia delle particelle accelerate in shock lievemente relativistici, questi risultati forniscono utili punti di riferimento osservativi per lo sviluppo della teoria dell'accelerazione delle particelle applicata al caso di emissione prompt. Il mio progetto di dottorato ha compreso anche lo studio dell'emissione a frequenze più basse dei primi 3 GRB rilevati ad energie ~TeV (GRB 180720B, GRB 190114C, GRB 190829A), che mi ha permesso di trovare interessanti vincoli sui parametri macro e micro-fisici dei GRBs.
The nature of the GRB prompt emission is still uncertain, preventing us from constraining the sources' physical properties, which are strictly connected to fundamental open issues such as jet composition and energy dissipation. The typical observed prompt emission spectrum consists of two power-laws with slopes α and β smoothly connected at a peak energy Epeak. Synchrotron from fast cooling electrons is the most natural radiative process, given the non-thermal shape of the observed spectrum and the strong magnetic fields expected in the emitting region. However, the observed spectra of thousands of GRBs being harder than synchrotron predictions represented a major issue for decades. Oganesyan et al. 2017 recently discovered in the soft X-rays the presence of an additional spectral break, which has been identified as the synchrotron cooling frequency. Motivated by this result, I searched for the presence of a break at higher energies, using Fermi/GBM data. The time-resolved analysis of the 10 long brightest GRBs has revealed in the ~70% of the spectra the presence of a break energy Ebreak, located between ~20 and 500 keV. The slopes of the power-laws below and above the break are remarkably consistent with the predicted values for synchrotron emission in fast cooling regime (-2/3 and -3/2, respectively). Spectral simulations suggest that the separation of Ebreak from Epeak and the photon statistics of the spectrum can hamper the identification of the break, which might explain why this fundamental feature has not been identified before. The consistency with synchrotron emission has been tested also fitting a physical synchrotron model to the spectrum of the long GRB 180720B, confirming the results obtained with the empirical function. In addition, I investigated, for the first time, the presence of the break in 10 short GRBs: contrary to long ones, short GRBs do not show the break, but the low-energy photon index is consistent with -2/3. The results presented in this thesis imply a set of physical parameters that challenges the GRB standard model. The relative small ratio of the synchrotron frequencies (closer to unity for short GRBs) implies that the emitting particles do not cool completely. In turn this requires a low magnetic field (B’≲10 G) implying a distant emission region (R~1016cm), at odds with the typical ~ms variability timescale of GRBs. A possible solution may come from the proton-synchrotron scenario: thanks to the longer cooling timescale of the protons, this scenario can explain the observed spectral shape assuming standard values both for the magnetic field (B’~106 G) and for the radius of the emitting region (R~1013cm). Moreover, I expanded my study of the prompt emission spectrum by including its characterization at higher energies. The addition of LAT data in the spectral analysis revealed in 10 out of 22 bursts the presence of a spectral cutoff at high energies (~100 MeV): interpreted as due to pair-production opacity, they provide estimates of the bulk Lorentz factor Γof the jet in the range 100-400. The extension at high energies allowed me also to study the high-energy power-law slope β, which is a key parameter to constrain the corresponding slopes p of the underlying energy distribution of non-thermal accelerated particles. Assuming the emission as due to synchrotron, I found a broad distribution of p, centered around p=2.86 (with a tail up to p~5-7). Given the theoretical uncertainties on the energy distribution of accelerated particles in mildly-relativistic shocks, these results provide useful observational benchmarks for the development of the theory of particle acceleration applied to the prompt emission case. My PhD project also included the study of the emission at lower frequencies of the first 3 GRBs detected at ~TeV energies (GRB 180720B, GRB 190114C, GRB 190829A), which allowed me to find interesting constraints on the macro- and micro-physical parameters of GRBs.

Gamma-ray bursts (GRBs) are brief and sudden explosions radiating most of their energy in the soft γ-ray band ( 100 keV). In the context of multimessenger astroparticle physics recent observations of GRBs provide an excellent benchmark for testing theoretical models of high energy emission mechanisms. Acceleration of hadrons in the engine is expected to produce high energy neutrinos and gamma-rays simultaneously via π±/π0 decays, thus reinforcing the motivation for coincident searches in km3 neutrino telescopes. The Waxman-Bachall spectra and the corresponding expected neutrino rates in IceCube are derived here for GRB090510 amd GRB090902B recently detected by the Fermi Large Area Telescope. The implications of the significant detection of deviations from the Band function fit in photon spectra and a model that explains these extra-components in terms of π0-decay photons are presented here and the relevance to neutrino astronomy is shown.

L'éventuelle découverte d'une émission de neutrinos en coïncidence avec un sursaut gamma donnerait des informations importantes sur les mécanismes impliqués par les modèles les plus prometteurs. L'un des plus récents et reconnus est le modèle photosphérique. Ce modèle est très prometteur car il peut expliquer certaines mesures de sursauts gamma aux propriétés non prédites par les modèles précédents. Il prédit l'émission de neutrinos détectables à plus basse énergie que les modèles précédents. Dans ce travail de thèse une méthode innovante a été proposée et des outils informatiques dédiés à cette méthode ont été développés afin d'améliorer la sensibilité du détecteur ANTARES dans la gamme en énergie prédite par le modèle photosphérique. De plus et pour la première fois, les données non filtrées, prises en coïncidence avec des sursauts gamma, ont été utilisées. Le déclencheur directionnel et l'algorithme de reconstruction dédié aux basses énergies développés dans le cadre de ce travail ont amélioré la sensibilité du détecteur ANTARES d'un facteur 2 (entre 100 GeV et 1 TeV), ce qui a démontré leur efficacité. Cette analyse a été appliquée a la recherche d'émission de neutrinos durant les sursauts gamma GRB130427A et GRB130505A. Aucune émission de neutrino n'a été détectée, mais une limite supérieure sur le flux de neutrinos a pu être établie dans la région d'intérêt en énergie des modèles étudiés
The possible evidence of neutrino emission in correspondence of a Gamma ray burst (GRB) would gave important informations on the production mechanisms and on the most promising emission models. Among these, one of the most recent and widely discussed is the so-called photospheric model. It can explain some measured features of the gamma ray bursts that are not described by the previous models. The photospheric model predicts a sensible neutrino emission in an energetic range a couple of order of magnitude lower with respect to the classical models. In this thesis a completely innovative method of research has been proposed and dedicated tools have been developed to enhance the ANTARES detector sensitivity in the energetic range predicted by the photospheric model. To this extent, for the first time, the unflitered data registered in ANTARES in correspondence of a gamma ray burst has been used. In addition a dedicated directional trigger and a special reconstruction algorithm optimized at low energies were found very effective. As a result, this analysis has enhanced the sensitivity of the dectector up to a factor 2 for neutino energies between 100 GeV and 1 TeV. This analysis was applied to the search of a possible neutrino emission in correspondance of the GRB130427A and GRB130505A events. No evidence for neutrino emission was found. However an upper limit on the neutrino flux from these GRBs in the energetic range which fats the predictions of the most recent emission models was determined

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.
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.

Les Sursauts de Rayonnement Gamma (GRBs) sont des flashs émanant du cosmos qui sont classés en deux groupes : les longs/doux et les courts/durs. Le lancement du satellite Swift a ouvert une nouvelle ère dans la recherche sur les GRBs. Cette thèse présente une étude détaillée du GRB 141221A qui montre une forte et inhabituelle hausse des valeurs des courbes de lumière dans le domaine optique lors de l'émission rémanente alors que dans le domaine des rayons X ces valeurs sont plus normales. GRB 141221A est un des GRBs qui mettent à l'épreuve les modèles car il exclut tout vent stellaire du progéniteur. Un regroupement des courbes de lumière dans le domaine X lors de l'émission rémanente GRBs a été observé avant le lancement de Swift. Cette classification a été débattue après le lancement de Swift. Nous avons construit un échantillon de 254 GRBs qui montre un éparpillement des distributions du flux à un jour. Cette distribution a été normalisée avec un décalage vers le rouge de 1. Nous avons investigué ce problème selon trois directions: un problème instrumental, un problème de calibration des données ou l'absence de regroupement. Coté instrument, le problème pourrait être observationnel, nous avons en effet observé des effets saisonniers durant les solstices et les équinoxes. Coté calibration des données, un tel problème pourrait avoir une influence sur les résultats de l'étude. La comparaison entre analyse manuelle et automatique des données telle que disponible sur le dépôt Swift-XRT montre de sérieux aléas sur les résultats. Les cas où l'analyse manuelle est judicieuse sont exposés. La dernière possibilité de l'absence de regroupement pourrait être un effet de sélection
GRBs are ashes of gamma-rays coming from cosmos. They are one of the most mysteriousevents we have been able to observe since their discovery. GRBs are classified into two groups: long/soft GRBs and short/hard GRBs. Their emission mechanism consists of two phases: prompt emission and afterglow emission. The launch of the Swift satellite opened a New Era in the GRBs research. Swift is able to provide accurate position for more GRBs than previous missions, thanks to its fast capabilities of slewing. Furthermore, the Swift shows that GRBs have a canonical behaviour for the X-ray afterglow light curves. The data analysis process remains the key point of GRBs studies. I present a detailed study of the peculiar GRB 141221A at different wavelengths. GRB 141221A shows an unusual steep rise in the optical light curve of the afterglow. The broad band spectral energy distribution, taken near the maximum of the optical emission, presents either a thermal component or a behaviourbreak. This burst displays unusual feature in the optical band, whereas the X-ray data is more common. GRB 141221A is one of the challenging bursts that excludes a stellar wind from the progenitor of the GRBs. A clustering in the X-ray afterglow light curves was observed before the launch of the Swift satellite. This feature has been debated after the launch of the Swift. We have built a sample which consists of 254 GRBs that shows a scattering of the data for the flux distribution at one day. This distribution was investigated with a normalization of light curves at redshift one. We have investigated the problem into three different directions

The IceCube Neutrino Observatory has reported the observation of 35 neutrino events above 30 TeV with evidence for an astrophysical neutrino flux using data collected from May 2010 to May 2013. These events provide the first high-energy astrophysical neutrino flux ever observed. The sources of these events are currently unknown. IceCube has looked for correlations between these events and a list of TeV photon sources including a catalog of 36 galactic sources and 42 extragalactic sources, correlations with the galactic plane and center, and spatial and temporal clustering. These searches have shown no significant correlations. The isotropic distribution of the event directions gives indications that the events could be extragalactic in nature and therefore may originate in the same processes that generate ultra-high-energy cosmic rays (UHECRs). The sources of these UHECRs are still unknown; however, gamma-ray bursts (GRBs) have been proposed as one possible source class. By determining the source of these high-energy neutrinos, it may be possible to determine the sources of UHECRs as well. This study is a search for directional and temporal correlation between 856 GRBs and the astrophysical neutrino flux observed by IceCube. Nearly 10,000 expanding time windows centered on the earliest reported time of the burst were examined. The time windows start at ±10 s and extend to ±15 days. We find no evidence of correlations for these time windows and set an upper limit on the fraction of the astrophysical flux that can be attributed to the observed GRBs as a function of the time window. GRBs can contribute at most 12% of the astrophysical neutrino flux if the neutrino-GRB correlation time is less than ≈20 hours, and no more than 38% of the astrophysical neutrino flux can be attributed to the known GRBs at time scales up to 15 days. We conclude that GRBs observable by satellites are not solely responsible for IceCube’s astrophysical neutrino flux, even if very long correlation time scales are assumed.

In questo lavoro di tesi si svilupperanno simulazioni col software BoGEMMS, il quale si basa sulle librerie del toolkit Geant4 del CERN, che simula il trasporto di particelle attraverso la materia. Successivamente si studieranno le caratteristiche dei principali sottosistemi del payload del satellite e-ASTROGAM, proposto per la call M5 dell’ESA, e le componenti di rumore che sono presenti in un sistema di rivelazione, incentrando l’attenzione sugli effetti che comportano il charge sharing e il rumore elettronico sulla determinazione della risoluzione spaziale e della PSF di e-ASTROGAM. Esso è il primo satellite gamma in grado di coprire il range energetico 300 keV−3 GeV, che finora è rimasto in parte inesplorato poichè interessa un intervallo in energia in cui la radiazione viene emessa sia per effetto Compton sia tramite produzione di coppie, rendendo di fatto complessa la rivelazione del segnale derivante da entrambi i processi con un unico strumento. Grazie all’ampio campo di vista, l’ottima risoluzione angolare e la buona PSF, e-ASTROGAM sarà in grado di rivelare un gran numero di sorgenti transienti, tra le quali si stima un tasso di rivelazione di ∼600 Gamma-Ray Burst (GRB) entro i primi tre anni dal lancio, programmato per il 2029. In particolare gli short-GRB sono ottimi candidati a ricoprire il ruolo di controparte elettromagnetica delle onde gravitazionali (GW, Gravitational Wave) in quanto sia i GRB sia le GW sono stati associati al merging di oggetti compatti. Per questo motivo, tramite e-ASTROGAM, ci si aspetta di rivelare negli anni a venire dei fenomeni transienti simultanei o quasi simultanei nell’intervallo energetico coperto dal satellite per avvalorare l’associazione GW−sGRB.

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.

We discuss the evolution of a relativistic outflow responsible for producing the emission associated with GRBs. We investigate how afterglows are produced in the inter- action between the outflow and the ambient medium. Understanding the properties of the outflow from afterglow emission can be coupled with information obtained from the prompt component to constrain the magnetisation of the outflow. We analytically and numerically evaluate the relative strength of the reverse shock emission as the out- flow propagates into either a wind or ISM -type environment. We find that previous estimates of magnetisation based on the relative strength of forward and reverse shock emission had been underestimated by up to a factor of 100. We then apply our revised magnetisation estimate to a sample of 10 GRBs and find that 5 of the 10 events can be described by the ISM model. As recent studies have indicated that the fraction of en- ergy stored in the magnetic fields are small, our findings would suggest that the ejecta is driven by thermal pressure. Finally we consider how inhomogeneities present in the outflow can lead to variations in the very early afterglow. Considering small gradi- ent in the ejecta density profile modifies the rising index of the afterglow and can be equivalent to changing the dimensionless parameter ξ by a factor of 2. Uncertainties in determining the width of the ejecta present difficulties in understanding the distribution of GRBs afterglow rising index.

The aim of this thesis is to study Gamma-Ray Burst (GRB) progenitors and central engines, I begin by examining unexpected plateaus in GRB light curves and place constraints on the central engine, that are consistent with a proto-magnetar. Next I compare these to the normal plateaus seen in the light curve and expand my investigation to include flares. Finally I investigate whether some giant flares could actually be a GRB if the GRB in those light curves could be a progenitor.

This thesis firstly describes efforts to characterise large-area, high-gain, Avalanche Photodiodes (APDs), manufactured by Radiation Monitoring Devices (RMD) inc. of Massachusetts. These are relatively new devices in the field of X-ray spectroscopy and the research presented here attempts to increase our understanding of their behaviour as X-ray detectors and their underlying internal physical processes. Models are suggested for Quantum Detection Efficiency and for Photopeak Fraction in these devices. Measurements of these properties as a function of energy constrain the models, revealing new information about the internal structure of APDs and providing powerful predictive tools for detector response. The intrinsic silicon dead layer of a typical device is found to be 2 microm thick, whilst the sensing layer is 21 microm thick. Secondly, this thesis provides detailed accounts of how the new tools mentioned above have been utilised to characterise an ensemble of APDs and how that characterisation has been used to simulate the behaviour of an APD-based astrophysical instrument: CATSAT's Soft X-ray Spectrometer (SXR). This work includes simulated SXR observations of the diffuse soft X-ray background, the crab nebula and CATSAT's target objects: Gamma-ray Bursts. The results of these simulations are presented, leading to an analysis of CATSAT's ability to meet its scientific objectives. It is estimated that the SXR will observe approximately 7 bursts per year above the five-sigma significance level. In approximately half of these cases, it should be possible to discriminate between the hypothesis that there is an absorbing hydrogen column of density 1 x 1022 cm-2 and the hypothesis that there is no column.

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.

I Gamma Ray Burst (GRB) sono fenomeni complessi la cui origine ed i cui meccanismi fisici sono ancora oggetto di dibattito scientifico. Osservazioni multifrequenza dell’afterglow dei Gamma Ray Burst possono essere utilizzate per porre dei vincoli ai parametri fisici fondamentali dell’esplosione. Uno dei maggiori aspetti del successo nella ricerca dei GRB ´e dato dalla rapidit´a con cui il telescopio ´e in grado di puntare la regione del GRB. Lo scopo del mio lavoro ´e stato quello di studiare le prime fasi dei Gamma Ray Burst a differenti lunghezze d’onda per capirne meglio le propriet´a fisiche, anche in vista delle prospettive future nella rivelazione di fotoni ad alta energia dai GRB offerte dalla nuova generazione di strumenti. Al fine di studiare l’emissione ad alta energia e nelle bande infrarosse ho utilizzato dati collezionati dai telescopi terrestri MAGIC (per le alte energie) e REM (nelle banda infrarosse). Entrambi questi strumenti sono stati costruiti con l’intento di permettere un rapido riposizionamento (nell’ ordine delle decine di secondi) nel caso di un GRB. Grazie al progetto di questa tesi il primo catalogo dei GRB rivelati da REM sar´a presto disponibile. Il catalogo conterr´a dettagliate informazioni sulle posizioni, sulle propriet´a temporali e fotometriche di tutti gli oggetti. Utilizzando le propriet´a delle curve di luce nelle bande infrarosse, ottenute attraverso un’analisi di fitting dei burst con un’estesa copertura temporale, ho posto dei limiti su alcuni parametri fondamentali della fireball che possono essere utilizzati per discriminare tra diversi meccanismi al lavoro nei GRB. In particolare ho studiato in dettaglio il meccanismo di emissione da Synchrotron Self Compton (SSC) concludendo che grazie ai nuovi strumenti, in particolare con MAGIC ed i suoi recenti miglioramenti tecnologici, la rivelazione di fotoni emessi per SSC ´e possibile nel caso di un burst sufficientemente energetico ed a basso redshift. Nella prima parte di questa tesi, dopo un’introduzione sui Gamma Ray Burst (capitolo 1), introdurr´o il telescopio MAGIC (capitolo 2) e presenter´o due burst interessanti seguiti da MAGIC per i quali ho studiato l’emissione gamma nel GeV nel contesto del meccanismo di Synchrotron Self Compton: GRB 080430 e GRB 090102 (capitolo 3). La seconda parte ´e dedicata all’osservazione dei Gamma Ray Burst nelle bande infrarosse effettuata con il telescopio REM (capitolo 4). Per la prima volta sono stati analizzati tutti i GRB osservati da REM dall’inizio delle operazioni fino alla fine del 2009 (capitolo 5). Per alcuni burst ben campionati ´e stato possibile stimare l’epoca del picco dell’afterglow e quindi determinare il fattore di Lorentz ed il parametro e della fireball. Questi parametri sono stati utilizzati per ottenere una stima precisa del flusso atteso alle alte energie dal meccanismo di SSC. Infine queste previsioni teoriche sono state confrontate con i risultati di MAGIC.
Gamma-ray bursts (GRBs) are an enigmatic phenomenon whose physics and origins are still to be fully understood. Broadband observations of gamma-ray burst afterglows can in principle be used to constrain fundamental physical parameters of the explosion. One major aspect of successful GRB research is how fast the telescope can point the GRB region. The aim of my work was to investigate the early phases of GRB phenomenon at different wavelenghts to better understand their physical properties, also in view of the future perspectives offered by the new instruments in detecting the high energy photons from GRBs. In order to investigate high energy and infrared emission from GRBs I used data collected by the ground-based MAGIC Telescopes (at the high energies) and the REM telescope (in the near infrared band). Both instruments have been built in order to allow a rapid repositioning (tens of seconds). Thanks to the project of this thesis the first REM gamma -ray burst afterglow catalog will be avaiable soon. The catalogue will contain detailed burst positional, temporal and photometric information. Moreover, using the observed infrared properties obtained by the fitting analysis of those burst with an extended time-coverage, I put some constrains on fundamental fireball parameters that can be used to discriminate between different mechanism at work in GRBs. In particular I investigated the Synchrotron Self Compton (SSC) emission mechanism leading to the conclusion that thanks to the new incoming instruments, in particular with MAGIC and its new hardware upgrade, the detection of SSC emission from an energetic and low redshift burst is possible. In the first part of this thesis, after an introduction about the GRB phenomenon (chapter 1), I will introduce the MAGIC telescope (chapter 2) and I will present two interesting burst followed up by MAGIC for which I explored the GeV gamma-ray emission in the context of Synchrotron Self Compton mechanism (SSC): GRB 080430 and GRB 090102 (chapter 3). The second part is dedicated to infrared observations of GRBs performed by REM telescope (chapter 4). For the first time all the GRBs observed by REM, since the beginning of its operations up to the end of 2009, are analyzed (chapter 5). For some well sampled burst it was possible to evaluate the epoch of the afterglow peaks and thereby constrain the Lorentz factor and the e fireball parameters. These parameters obtained were used to derive meaningful prediction for the expected SSC flux at higher energy. Finally those predictions were compared with the MAGIC results.

I gamma-ray burst (GRB) sono brevi lampi di fotoni che vengono intercettati dagli odierni rivelatori spaziali di raggi X duri e di raggi gamma ogni due o tre giorni. Per un tempo che può durare da meno di un secondo a diverse migliaia di secondi, un flusso di fotoni altamente variabile, con una struttura temporale imprevedibile, investe il rivelatore. Sono trascorsi cinquant'anni dalla prima osservazione di questo tipo, durante i quali una lunga serie di avanzamenti tecnologici teorici ha lastricato la strada che ha condotto all'odierno paradigma secondo il quale questi lampi sono legati all'accrescimento di materia su un buco nero o una stella di neutroni appena nati. Due sono i luoghi di nascita naturali di queste bestie relativistiche: il collasso di una stella massiva o la coalescenza di due oggetti compatti. Il secondo processo, forse il più affascinante dei due, è stato il primo ad essere proposto come possibile progenitore dei GRB, ma nel 1998 l'associazione del GRB 980425 con la supernova 1998bw ha costituito la prima prova schiacciante del primo scenario. Nonostante ciò, nessuna supernova è stata ancora associata - in alcuni casi con dei limiti molto stringenti - ad un membro di una sottoclasse di questi eventi, quella degli short gamma-ray bursts (SGRB). Diversi indizi infatti supportano l'idea che il progenitore dei SGRB sia proprio la coalescenza di due stelle di neutroni, o di un buco nero e una stella di neutroni. Se questo dovesse essere vero, allora c'è un legame fondamentale tra SGRB e onde gravitazionali (GW). La rete di rivelatori terrestri avanzati di GW - che al momento consiste dei due Advanced LIGO negli Stati Uniti, e di Advanced Virgo in Italia - è sensibile in particolare alla banda di frequenze in cui sono emesse le GW prodotte dallo spiraleggiare e fondersi di oggetti compatti di massa stellare, per cui tutto è pronto per poter testare la connessione SGRB-GW. Nell'agosto di quest'anno, la prima osservazione di GW dalla coalescenza di due stelle di neutroni, seguita dall'osservazione in associazione di una kilonova - l'emissione ultravioletta, ottica e infrarossa proveniente da materiale in espansione, lanciato durante la fase di fusione e quella successiva alla fusione della coalescenza, la cui sorgente di energia è il decadimento radioattivo di nuclei instabili sintetizzati attraverso la cattura rapida di neutroni - e di un lampo simile ad un SGRB hanno segnato l'inizio di una rivoluzione, il cui effetto sulla nostra comprensione di questi fenomeni deve ancora dispiegarsi completamente. Per questa ragione, in questa tesi non traggo ferme conclusioni da queste osservazioni, ma piuttosto discuto alcune possibili interpretazioni e conseguenze, lasciando molte domande aperte a future investigazioni.
Gamma-ray bursts (GRBs) are brief flashes of photons that trigger current space-based hard X-ray and gamma-ray detectors every two or three days. During a time ranging from less than one to several thousand seconds, a highly variable photon flux with an unpredictable time structure is recorded by the detector. Fifty years have flown since the first observation of this kind, during which a long series of technological and theoretical breakthroughs paved the way for the current, widely-accepted paradigm that relates these flashes to accretion of matter on a newborn stellar-mass black hole or neutron star. Two are the natural birthplaces of such relativistic beasts: the collapse of a massive star and the coalescence of two compact objects. The latter, perhaps the most intriguing of the two, was the first to be proposed as a candidate progenitor of GRBs, but in 1998 the association of GRB 980425 with supernova 1998bw provided compelling evidence for the former. Nevertheless, no supernova has been associated so far – in some cases down to very stringent limits – to members of a particular subclass of these events, known as short gamma-ray bursts (SGRBs). Several pieces of evidence support the idea that the progenitor of SGRBs is indeed the coalescence of two neutron stars, or of a black hole and a neutron star. If this is true, then SGRBs are also intimately related to gravitational waves (GW). The advanced network of ground-based GW detectors – which at present consists of the two Advanced LIGO interferometers in the USA and of Advanced Virgo in Italy – is especially sensitive in the frequency range of GW produced by the inspiral and merger of a stellar mass compact object binary, so that we are right in the position to start testing the SGRB–GW connection. In August of this year, the first observation of GW from a neutron star binary coalescence, followed by the first observation of a kilonova – the UV/Optical/Infrared emission from the expanding material ejected during the merger and post-merger phases of the coalescence, powered by nuclear decay of unstable nuclei synthesized by the r-process – and an associated SGRB-like transient marked the start of a revolution, whose effect on our understanding of these subjects still needs to be completely unfolded. For this reason, in this thesis I do not to draw firm conclusions about these observations, but rather I discuss some possible interpretations and implications, leaving many questions open to future investigation.

Les sursauts gamma (GRB) sont de brèves bouffées de rayonnement électromagnétique à haute énergie provenant de l'espace, formés dans un jet ultra-relativiste à l'issue de la naissance d'un trou noir stellaire. Une sous-classe de GRB dite longs (LGRB) s'est avérée associée à l'effondrement de coeur d'étoiles massives, ainsi les LGRB ont été suggérés comme des traceurs prometteurs de la formation stellaire jusqu'aux premiers ages de l'Univers. Cependant, les coeurs stellaires ne donnent pas tous lieu à des LGRB, car ces derniers sont des événements rares nécessitant des conditions particulières pour être formés. Dans cette thèse, nous étudions un ingrédient essentiel pour utiliser les LGRB en tant que traceurs de formation stellaire: l’efficacité des LGRB, définie comme la fraction des effondrements de coeur qui forme un LGRB. Dans la première partie de la thèse, nous présentons un modèle statistique de population développé pour reproduire trois contraintes observationnelles soigneusement sélectionnées. Ce modèle nous permet de contraindre la population intrinsèque de LGRB et de montrer que l'efficacité des LGRB augmente avec le redshift. La deuxième partie de la thèse est consacrée à l'identification de facteurs affectant l'efficacité des LGRB par l'étude d'un échantillon complet et sans biais de LGRB à 1 < z < 2. Nous étudions les environnements dans lesquels se forment les LGRB en étudiant les propriétés de leurs galaxies hôtes et en les comparant aux galaxies à formation d'étoiles. Nous montrons que la métallicité est un facteur déterminant régissant l’efficacité des LGRB. Ces résultats combinés offrent un pas de plus vers la compréhension des astres parents des LGRB
Gamma-ray bursts (GRBs) are brief bursts of high-energy electromagnetic radiation originating from space, formed in an ultra-relativistic jet following the aftermath of a newly born stellar mass black hole. A subclass of so-called long-duration GRBs (LGRBs, lasting typically a few tens of seconds) have been shown to be associated with the core-collapse of massive stars. Due to the short-lived nature of massive stars, LGRBs have been suggested as promising tracers of star formation up to the earliest epochs of the Universe. However, not all massive star core-collapses give rise to LGRBs as these are rare events, requiring particular conditions to form. In this thesis, we investigate a crucial ingredient for using LGRBs as tracers of star formation: the LGRB efficiency, defined as the fraction of core-collapses that form an LGRB. In the first part of the thesis, we present a statistical population model developed to reproduce three carefully selected observational constraints. This model allows us to constrain the intrinsic LGRB population and quantify the LGRB efficiency, showing that it increases with redshift. The second part of the thesis is devoted to identifying the factors affecting the LGRB efficiency by studying a complete, unbiased sample of LGRBs at 1 < z < 2. We study the environments in which LGRBs form by studying the properties of their host galaxies and comparing them with typical star-forming galaxies. We show that, as expected from theoretical considerations, metallicity is a key driving factor behind the LGRB efficiency. These results combined bring us one step closer to understanding the progenitors of LGRBs

The merger of a neutron star (NS) binary may result in the formation of a rapidly spinning magnetar. The magnetar can potentially survive for seconds or longer as a supramassive NS before collapsing to a black hole if, indeed, it collapses at all. During this process, a fraction of the magnetar's rotational energy of similar to 10(53) erg is transferred via magnetic spin-down to the surrounding ejecta. The resulting interaction between the ejecta and the surrounding circumburst medium powers a year-long or greater synchrotron radio transient. We present a search for radio emission with the Very Large Array following nine short-duration gamma-ray bursts (GRBs) at rest-frame times of approximate to 1.3-7.6 yr after the bursts, focusing on those events that exhibit early-time excess X-ray emission that may signify the presence of magnetars. We place upper limits of less than or similar to 18-32 mu Jy on the 6.0 GHz radio emission, corresponding to spectral luminosities of less than or similar to(0.05-8.3) x 10(39) erg s(-1). Comparing these limits to the predicted radio emission from a long-lived remnant and incorporating measurements of the circumburst densities from broadband modeling of short GRB afterglows, we rule out a stable magnetar with an energy of 10(53) erg for half of the events in our sample. A supramassive remnant that injects a lower rotational energy of 10(52) erg is ruled out for a single event, GRB 050724A. This study represents the deepest and most extensive search for long-term radio emission following short GRBs to date, and thus the most stringent limits placed on the physical properties of magnetars associated with short GRBs from radio observations.

The radiative mechanism responsible for the prompt gamma-ray burst (GRB) emission remains elusive. For the last decade, optically thin synchrotron emission from shocks internal to the GRB jet appeared to be the most plausible explanation. However, the synchrotron interpretation is incompatible with a significant fraction of GRB observations, highlighting the need for new ideas. In this thesis, it is shown that the narrow, dominating component of the prompt emission from the bright GRB090902B is initially consistent only with emission released at the optically thick jet photosphere. However, this emission component then broadens in time into a more typical GRB spectrum, which calls for an explanation. In this thesis, a previously unconsidered way of broadening the spectrum of photospheric emission, based on considerations of the lateral jet structure, is presented and explored. Expressions for the spectral features, as well as polarization properties, of the photospheric emission observed from structured, relativistic jets are derived analytically under simplifying assumptions on the radiative transfer close to the photosphere. The full, polarized radiative transfer is solved through Monte Carlo simulations, using a code which has been constructed for this unique purpose. It is shown that the typical observed GRB spectrum can be obtained from the photosphere, without the need for additional, commonly assumed, physical processes (e.g. energy dissipation, particle acceleration, or additional radiative processes). Furthermore, contrary to common expectations, it is found that the observed photospheric emission can be highly linearly polarized (up to $\sim 40 \, \%$). In particular, it is shown that a shift of $\pi/2$ of the angle of polarization is the only shift allowed by the proposed model, consistent with the only measurement preformed to date. A number of ways to test the theory is proposed, mainly involving simultaneous spectral and polarization measurements. The simplest measurement, which tests not only the proposed theory but also common assumptions on the jet structure, involves only two consecutive measurements of the angle of polarization during the prompt emission.

QC 20131204

We have performed a blind search for a gamma-ray transient of arbitrary duration and energy spectrum around the time of the LIGO gravitational-wave event GW150914 with the six-spacecraft interplanetary network (IPN). Four gamma-ray bursts were detected between 30 hr prior to the event and 6.1 hr after it, but none could convincingly be associated with GW150914. No other transients were detected down to limiting 15-150 keV fluences of roughly 5 x10-(8) -5 x 10(-7) erg cm(-2). We discuss the search strategies and temporal coverage of the IPN on the day of the event and compare the spatial coverage to the region where GW150914 originated. We also report the negative result of a targeted search for the Fermi-GBM event reported in conjunction with GW150914.