Rozprawy doktorskie na temat „Black hole X-ray binaries”

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

Black hole X-ray binaries (BHXBs) are stellar binary systems consisting of a black hole (BH) and a companion star. They are known to produce X-ray emission through the accretion of mass from the companion star onto the black hole via an accretion disc, as well as radio emission originating from their jets. My thesis splits into two projects. On one hand, I focus on the connection between the X-ray emitting accretion disc and the radio jets of BHXBs in general, by studying the quasi-simultaneous evolution of the radio fluxes and the X-ray fluxes from 17 BHXBs. This connection, also known as the radio/X-ray correlation has been studied and updated over the past years. New observations of new and known sources have shown that another population of X-ray binaries exists (referred to as outliers), lying below the standard radio/X-ray correlation. I investigate whether the mass of the black hole component of BHXBs can explain the existence of these outliers. In my second project, I focus on an exotic source, known as SS433. It has a supercritical accretion disc and displays precessing relativistic jets. I investigate whether these jets are made up of proton-electron plasma or electron-positron plasma. Circular polarization (CP) is a good diagnostics for understanding the particle composition of radio jets. Therefore we have observed the circular polarized flux densities of SS433 using the Australia Telescope Compact Array (ATCA) for a broad range of frequencies between 1:4 - 10 GHz. From those observations, a CP spectrum can be constructed and the spectral index can be estimated. There are 4 ways of producing CP emission and the spectral index helps us to constrain the CP production mechanism. In addition, the kinematics of propelling a proton-electron plasma in a jet is different from that of electronpositron plasma. I simulate various plausible models for the energy content of the jets and thereby aim to constrain the particle composition of the jets. Black hole X-ray binaries (BHXBs) are stellar binary systems consisting of a black hole (BH) and a companion star. They are known to produce X-ray emission through the accretion of mass from the companion star onto the black hole via an accretion disc, as well as radio emission originating from their jets. My thesis splits into two projects. On one hand, I focus on the connection between the X-ray emitting accretion disc and the radio jets of BHXBs in general, by studying the quasi-simultaneous evolution of the radio fluxes and the X-ray fluxes from 17 BHXBs. This connection, also known as the radio/X-ray correlation has been studied and updated over the past years. New observations of new and known sources have shown that another population of X-ray binaries exists (referred to as outliers), lying below the standard radio/X-ray correlation. I investigate whether the mass of the black hole component of BHXBs can explain the existence of these outliers. In my second project, I focus on an exotic source, known as SS433. It has a supercritical accretion disc and displays precessing relativistic jets. I investigate whether these jets are made up of proton-electron plasma or electron-positron plasma. Circular polarization (CP) is a good diagnostics for understanding the particle composition of radio jets. Therefore we have observed the circular polarized flux densities of SS433 using the Australia Telescope Compact Array (ATCA) for a broad range of frequencies between 1:4 - 10 GHz. From those observations, a CP spectrum can be constructed and the spectral index can be estimated. There are 4 ways of producing CP emission and the spectral index helps us to constrain the CP production mechanism. In addition, the kinematics of propelling a proton-electron plasma in a jet is different from that of electron-positron plasma. I simulate various plausible models for the energy content of the jets and thereby aim to constrain the particle composition of the jets.

The accretion of matter onto black holes results in their characteristic spectrum through which we can identify them and study their properties. Furthermore, this radiation can couple to their surroundings, resulting in complex interactions between black holes and their environments. In this thesis, I study the accreting properties of stellar mass black holes, and examine the effect that such interactions may have had on the early universe. I also consider the observational characteristics of the lowest luminosity stellar mass black hole binary systems in our own galaxy. Approximately one billion years after the Big Bang, the universe underwent a huge baryonic phase change, in which neutral hydrogen became ionized by the first sources of radiation. Massive stars are thought to drive this process, but their ionizing lifetimes could have been extended by a later phase in their evolution: black hole X-ray binary formation. However, the extent of this enhancement is not known, and has been highly debated in recent literature. In this thesis, I show that X-ray binaries were unlikely to be present in sufficient numbers to exert a significant effect on the intergalactic medium. Using a stellar population synthesis model of a single starburst event, I show that radiation from X-ray binaries dominates the ionizing power of a cluster after the most massive stars have ended their lives. However, their high energy spectra and short lifetimes mean their ionizing timescales are too long for them to affect the progress of reionization. Even so, the high escape fraction of X-rays from galaxies still provides scope for low level heating and ionization of the distant intergalactic medium under different circumstances, such as in the context of continuous star formation. I also assess the detectability of the dimmest black hole binary systems in the Milky Way. Using a catalogue of black hole binaries in our galaxy, I find that there is a statistically significant lack of short orbital period systems, when compared to the neutron star binary population. I show that these sources may be hidden from view, rather than being truly absent, due to radiatively inefficient accretion, in which energy is lost to the black hole. However, this conclusion requires that the switch to inefficient accretion occurs sharply at a threshold mass accretion rate. In the case of a smoother switch, alternative observational or evolutionary arguments must be put forward to explain this dearth.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.
Includes bibliographical references (p. 183).
In rare cases, optical observations of Galactic binary star systems which are bright in the X-ray portion of the electromagnetic spectrum dynamically constrain the mass of one component to be well above theoretical limits for a neutron star. These systems - and systems with similar X-ray properties - are classified as black hole binaries. In this thesis, I report on observations of black hole binaries made with satellite observatories in the X-ray band. The region closest to the black hole is revealed in X-rays due to the viscous heating of matter that is accreted from the companion star. X-ray observations of these systems may therefore reveal General Relativistic effects. A fundamental and testable prediction of General Relativity is that matter may orbit more closely around black holes with significant angular momentum. I have investigated the possibility of black hole "spin" and the geometry of accretion flows in these systems using X-ray continuum spectroscopy, fast variability studies, and the shape of iron fluorescent emission lines in this band. I present evidence for black hole spin in XTE J1550-564, XTE J1650-500, and XTE J1748-248. Spin is not required by high-resolution spectral analysis of the archetypical Galactic black hole - Cygnus X-1 but a thermal accretion disk plus hot corona geometry is confirmed. Studies of XTE J1118+480 and GRS 1758-258 at low X-ray luminosity reveal that models for radiatively-inefficient accretion do not satisfactorily describe the geometry in these systems.
y Jon Matthew Miller.
Ph.D.

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.

This thesis presents research into the X-ray binary population of NGC 5128 (Centaurus A). The two principle investigations focus on the identification of black hole candidates, which can be identified by their long term variability and spectral properties. We demonstrate this with what we believe is our best example; a source that faded over two months of observations and displayed cool disc thermal-dominant spectra when at high luminosities- similar to the Galactic black hole X-ray binaries. The main result of this research is that the population of black hole X-ray binaries is more pronounced in the dust lane of the galaxy compared to in the halo. The explanation of this result, based around the mass of the donor stars required for systems to emit at the observed luminosities, may also explain the long noted effect of a steepening of the X-ray luminosity function in early-type galaxies at a few 10٨38 erg/s; an effect that increases with the age of the stellar population. Finally, frequent Chandra observations of the NGC 5128 were used to investigate the two known ultraluminous X-ray sources. These are transient systems and were observed at luminosities (1-10)% of their peak, in the regime frequented by the Galactic X-ray binaries. This presented an exciting opportunity to study the lower luminosity behaviour of these systems in an effort to determine the mass of the accreting compact object. The results of the spectral analysis point towards accretion powered by a stellar, rather than intermediate mass black hole. The long term variability of these sources is reminiscent of several of the long period Galactic X-ray binaries.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.
Includes bibliographical references (p. 97-100).
The power density spectrum is an essential tool for determining the frequency content of X-ray radiation from astronomical sources. For neutron star systems, power density spectra reveal coherent oscillations for those sources that are pulsars, while quasi-periodic oscillations over a wide range of frequencies (0.01 to 1300 Hz) are used to identify subclasses and to probe the details of accretion physics. For black hole binaries, the power density spectrum is useful in many important contexts: distinguishing black hole binaries from neutron star binaries, tracking the evolution of X-ray states, and understanding the dynamics of accretion disks, in particular the high-frequency oscillations that appear to be rooted in general relativity for strong gravitational fields. However, measurements of the power density spectrum are modified by the effects of deadtime in X-ray detectors. In this work, we focus on the Proportional Counter Array (PCA) instrument of the Rossi X-ray Timing Explorer (RXTE), an orbiting observatory that offers fast, microsecond-level time resolution and modest spectral resolution for celestial X-ray sources. We derive a new model for the effect of detector deadtime on measurements of the power density spectrum.
(cont.) The model treats in a unified manner the contributions from self-deadtime among selected events and interference from non-selected events. Using high-frequency power density spectra obtained from observations of X-ray sources, the new model is shown to be more accurate than existing approaches. The comparison between the model and the observations leads to a measurement of 8.83 s for the fundamental instrument deadtime timescale, which is dominated by the analog-to-digital conversion time. We additionally measure 59 jts and 137 /is for the Very Large Event deadtime related to observer-specified settings 1 and 2 respectively. Future refinements to the deadtimle model are discussed, such as corrections for highly variable sources and for individual X-ray energy bands.
(cont.) A preliminary comparison between power density spectra from black hole binaries and neutron star binaries is undertaken using the new deadtime model. While it may be possible to use high-frequency cut-offs in the power continuum to distinguish neutron star binaries from black hole binaries in the thermal and hard X-ray states, the comparison is inconclusive for black hole binaries in the steep power-law state. Since state definitions require considerations of X-ray spectral properties, the comparison results dispute a suggestion in the literature that accreting neutron stars and black holes can be distinguished on the basis of power density spectra alone.
by Dennis Wei.
S.B.

High-mass X-ray binaries (HMXRBs), such as Cygnus X-1, host some of the most rapidly spinning black holes (BHs) known to date, reaching spin parameters a greater than or similar to 0.84. However, there are several effects that can severely limit the maximum BH spin parameter that could be obtained from direct collapse, such as tidal synchronization, magnetic core-envelope coupling, and mass loss. Here, we propose an alternative scenario where the BH is produced by a failed supernova (SN) explosion that is unable to unbind the stellar progenitor. A large amount of fallback material ensues, whose interaction with the secondary naturally increases its overall angular momentum content, and therefore the spin of the BH when accreted. Through SPH hydrodynamic simulations, we studied the unsuccessful explosion of an 8 M-circle dot pre-SN star in a close binary with a 12 M-circle dot companion with an orbital period of approximate to 1.2 days, finding that it is possible to obtain a BH with a high spin parameter a greater than or similar to 0.8 even when the expected spin parameter from direct collapse is a less than or similar to 0.3. This scenario also naturally explains the atmospheric metal pollution observed in HMXRB stellar companions.

Black hole binaries (BHBs) consist of a black hole which accretes matter from a companion star and emits radiation primarily in the X-ray band. They are known to evolve through various states of emission, which are believed to signify changes in the accretion geometry. MAXI J1659--152 is a recently discovered galactic BHB, and we used Rossi X-ray Timing Explorer (RXTE) observations to investigate its state evolution during its 2010 outburst. This evolution was found to be similar to that of other known BHBs, although its thermal spectral component was relatively weak. The data was also used to estimate the black hole mass to be 3.6--8.0 solar masses. Archival RXTE and Swift data were used to examine another BHB known as GX 339--4 in its faint, hard emission state. This source has a persistent iron line in its spectrum throughout its various emission states, and it is frequently used to estimate the inner radius of its accretion disk. The data were unable to constrain the inner radius through the modeling of the iron line, but estimates based on modeling of the thermal spectral component proved to be consistent with an increase in inner radius at low luminosities. Theoretical predictions of the soft X-ray polarization of BHBs indicate a change in both angle and magnitude with energy. The details of this change depend on both the spin and mass of the black hole. The NASA Gravity and Extreme Magnetism Small Explorer (GEMS) mission sought to use this effect to measure the spin of BHBs, which is necessary to develop tests of the Kerr metric in general relativity. The Bragg Reflection Polarimeter (BRP) was the student experiment on this mission, and was in the beginning stages of flight fabrication at the time of the mission's cancellation in May 2012. A prototype multilayer reflector meeting nearly all requirements was developed and its performance measured at a synchrotron beamline. Monte-Carlo simulations were carried out to estimate the ultimate polarization sensitivity of the BRP, and indicated satisfaction of the BRP science requirement. Finally, a fully polarized, 511 eV beamline was developed and used to calibrate a BRP instrument prototype, validating the sensitivity predictions.

I buchi neri transienti (BHT) sono tra le sorgenti con emissione ai raggi X più luminose della galassia. Grazie all’elevato flusso in banda X e alla loro alta variabilità temporale. queste sorgenti offrono un’opportunità unica per studiare la fisica dell’accrescimento in straordinareie condizioni fisiche. I BHT mostrano episodici outburst caratterizzati da diverse luminosità in banda X e γ, diverse forme spettrali e proprietà della variabilità temporale. L’obiettivo di questa tesi è lo studio della geometria, dei meccanismi e dei processi fisici coinvolti nell’emissione del buco nero transiente GRS 716-249. Di seguito presento l’analisi spettrale e temporale delle osservazioni della GRS 1716-249 ai raggi X effettuate con il satellite Neil Gehrels Swift bservatory durante l’outburst verificatosi nel 2016-2017. Questi dati mi hanno permesso di studiare l’evoluzione dei parametri fisici durante tutta la durata dell’outburst e di studiare come varia la geometria della materia in accrescimento attraverso le transizioni spettrali. In particolare, coerentemente con lo scenario del disco di accrescimento troncato in cui il disco si avvicina all’oggetto compatto durante l’evoluzione dell’outburst, ho osservato che il disco di accrescimento della GRS 1716-249 potrebbe aver raggiunto l’ultima orbita stabile mentre la sorgente si trovava nello stato hard intermedio. Grazie al monitoraggio radio effettuato durante l’outburst ho potuto localizzare la sorgente sulla sempre più popolata correlazione radio/X degli "outliers" (o radioquieti) nel piano delle luminosità radio/X. Successivamente, mi sono concentrata sull’emissione ai raggi X/γ della sorgente. Questo mi ha permesso di osservare un eccesso nell’emissione alle alte energie, sopra a 200 keV, in aggiunta allo spettro di Comptonizzazione termica, nello spettro della GRS 1716-249. L’origine di questa componente può essere dovuta a processi di Compton inverso tra i fotoni soft del disco d’accrescimento e una popolazione di elettroni non-termici nella corona, o all’emissione di sincrotrone prodotta dagli elettroni energetici nel getto. Inizialmente ho modellando lo spettro X/γ della sorgente con modelli ibridi di Comptonizazione termica/non-termica: EQPAIR e BELM. In particolare, utilizzando BELM ho potuto stimare un limite superiore sull’intensità del campo magnetico nella corona. Infine, ho considerato la possibilità che l’eccesso di energia alle ate energie sia dovuto all’emissioni del jet. A tale scopo, ho prodotto la distribuzione d’energia spettrale della GRS 1716-249 usando le osservazioni multi-banda (dalla banda radio ai raggi γ) eseguite quando la sorgente era nello stato hard. Il flusso di accrescimento l’ho modellato con un modello di disco irradiato unito ad un modello di Comptonizzazine, mentre l’emissione del getto l’ho modellata con il modello Internal Schock Emission Model (ISHEM). Questo modello assume che le fluttuazioni di velocità del getto siano guidate dalla variabilità delle proprietà temporali del disco di accrescimento. Sebbene (ISHEM riproduce i dati radio e soft γ della sorgente GRS 1716-249, i risultati favoriscono lo scenario di Comptonizazione non termica nel flusso di accrescimento rispetto all’emissione di sincrotrone del getto oltre 200 keV.
Black hole transients (BHTs) are among the brightest X-ray sources in the Galaxy. Thanks to their high X-ray flux and short variability time scales they offer a unique opportunity to study the physics of the accretion under extraordinary physical conditions. These sources show episodic outbursts characterised by different X/γ-ray luminosities, spectral shapes and timing properties. The aim of this thesis is the understanding of the geometry, mechanisms and physical processes playing a role in the bright black hole X-ray transient GRS 1716-249. I present the spectral and timing analysis of X-ray observations performed with the Neil Gehrels Swift Observatory on GRS 1716-249 during the 2016-2017 outburst. These data gave me the opportunity to study the evolution of physical parameters and geometry variation of the accreting matter through the spectral transitions during the whole outburst. I found that the accretion disc could have reached the inner stable circular orbit during the hard intermediate state, coherently with the truncated accretion disc scenario in which the disc moves closer to the compact object. Then, the radio monitoring performed during the outburst let me locate the source on the ever more populated radio-quiet branch on the radio/X-ray luminosity plane. Thereafter, focusing on the soft γ-ray emission of the source, I observed a high energy excess, above 200 keV, in addition to the thermal Comptonisation spectrum. This component could be originate either through inverse Compton of the soft photons by non-thermal electrons in the corona, or from synchrotron emission of energetic electrons in the jet. First, I fitted the broad band X/γ-ray spectrum of GRS 1716-249 with hybrid Comptonisation thermal/non-thermal models: EQPAIR and BELM. Using BELM I obtained an upper limit on the magnetic field intensity in the corona. Finally, I investigated the possible origin of this high energy excess as due to jet emission. To this aim, I computed the Spectral Energy Distribution of GRS 1716-249 with the multi-wavelength observations (from the radio band to γ-rays) performed. I modelled the accretion flow with an irradiated disc plus Comptonisation model and the jet emission with the internal shock emission model (ISHEM). This model assumes that the jet velocity fluctuations are directly driven by the variability of X-ray timing proprieties of the accretion flow. Even though ISHEM reproduces the radio and soft γ-ray data of GRS 1716-249, the results seem to disfavour the jet scenario for the excess above 200 keV, in favour of non-thermal Comptonisation process.

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.

It is now widely known that most of the large galaxies we observe (e.g. the Milky Way) host in their center a supermassive black hole ($10^{6}-10^{9}$ $M_\odot$). Several relationships between the central black hole mass and the properties of the stars in the central part of the galaxy have been established in the past 3 decades indicating that the central black hole is able to efficiently structure the matter around it due to episodes of accretion of matter onto the black hole. Recent infrared and optical sky surveys have detected supermassive black holes with masses around $10^{8-9}$ $M_\odot$ when the universe was less than a tenth of its current age and current theories have difficulties explaining how such massive objects could have formed over such short timescales. The goal of the present work is to shed light on the properties of a still largely unknown extreme accretion regime, the so called super-Eddington accretion regime. If such accretion regime could be sustained over sufficient timescales, it could play an important role in both the rapid growth of supermassive black holes as well as its co-evolution with its host galaxy. The aim of this work is therefore to apply high resolution spectroscopy to Ultraluminous X-ray sources in order to identify narrow spectral features to derive constrains on the outflows expected from super-Eddington accreting sources using data from the XMM-Newton observatory. For this purpose I developed a framework to analyse low count background dominated spectra that uses a Monte Carlo approach to detect these narrow features. After analysis of the source Holmberg II X-1, I identify 7 unresolved discrete features with a 3$\sigma$ confidence level that can be tentatively identified with ionic species. Furthermore, the instrumental resolution allows us to put upper limits on the broadening of the lines. This findings will allow us to probe the properties of the outflows of the super-Eddington regime and by extending the analysis to other sources we will able to characterize the observational properties of this accretion regime.

The detailed knowledge of the temporal behaviour of astrophysical objects is one of the main sources of information about physical processes occurring in several classes of objects. In this PhD thesis we investigated two different astrophysical topics, both of them linked to timing. We present the scientific analysis of the data collected from the Crab pulsar by means of the novel optical extremely fast-photon counters Aqueye and Iqueye, that have the best temporal resolution ever achieved in the optical domain (hundreds of picoseconds). Aqueye (Barbieri et al. 2008, 2009) was designed to be mounted at the Copernico telescope in Asiago. Iqueye (Naletto et al. 2009, 2010) is an improvedversion for the NTT telescope. Here we also discuss some ideas on modelling the millisecond variability observed in the X-ray flux from Low Mass X-ray Binaries with either a neutron star or a black hole. The timing analysis of the optical emission from the Crab pulsar requires to time-tag with extreme precision the photons as collected by an inertial observer. Therefore we must refer the time of arrival of photons (TOAs) to a reference frame that approximates an inertial frame to the level of precision needed. One usually refers TOAs to a reference system located at the solar system barycenter. Tempo2 (Hobbs et al. 2006, Edwards et al. 2006) is a software meant to model with extreme precision (up to 1 ns) TOAs as collected by an inertial observer. After baricentering TOAs a numerical code making use of a standard template was used to determine the phase of the mean peak of the Crab pulsar profile. By studying the phase behaviour it is possible to extract information about both the rotational period of the fast rotating neutron star and its derivatives. Moreover, the analysis of the phase-residuals left out after subtracting the standard pulsar timing model may reveal interesting features of the pulsar and its surroundings. With the anlysis of the residuals one can also check for possible discrepancies on the modelling. If some systematic residuals show up, then it is interesting to investigate the physical origin. Just to quote a few noticeable examples, we mention the discovery of the first extrasolar planetary system around the pulsar PSR B1257+12, obtained from the analysis of pulsar phase-residuals (Wolszczan & Frail 1992; Wolszczan 1994; Konacki & Wolszczan 2003). Other foundamental results deal with tests of General Relativity theory (Helfand et al. 1980; Kramer et al. 2006). Moreover, pulsar timing is now being planned as a tool to reveal gravitational wave (Stappers et al. 2006; Manchester 2010). The analysis of the optical phase-residuals of the Crab pulsar we performed has revealed poor corrections in the Roemer delay due to the Tempo2 configuration files. After correcting for them we can conclude that the rotational periods of the Crab pulsar measured by Aqueye/Iqueye agree with those quoted in the Jodrell Bank radio archive up to a few picoseconds. The TOAs from a photon-counter usually are affected by noise that obeys the Poisson statistics. We noticed possible discrepancies between the radio and optical rotational periods larger than the estimated Poissonian error, but a more extensive analysis of the pulsar timing noise and related errors is needed before any definitive conclusion can be drawn. We were able to measure the spin down of the neutron star already over a baseline of a few days. Discrepancies with that reported in the Jodrell Bank radio archive are underlined. By comparing the time of arrival of the optical peak at the solar system barycenter with that quoted in the Jodrell Bank radio ephemerides archive we find a radio-optical delay in agreement with that reported in the literature (Shearer et al. 2003; Oosterbroek et al. 2008), that is, an optical peak leading the radio one by about 120 microsec. We also noticed same radio-optical phase drift with time, which may be related to the radio-optical rotational period discrepancies mentioned above. A further investigation on the possible origin of these discrepancies led to the preliminary conclusion that the signal from the Crab pulsar may be affected by an extra-noise component, known as timing noise, not suitable described by the Poissonian statistics. Non-Poissonian noise in the signal from neutron stars has been reported by other authors (Boynton et al. 1972; Lyne et al. 1993; Scott et al. 2003; Hobbs et al. 2006b; Patruno et al. 2009), but using integration times of months or years. Further observations to confirm the existence of non-Poissonian noise in the Crab pulsar are needed. In this PhD thesis we also present some ideas on the origin of the millisecond X-ray timing variability in the X-ray flux from Low Mass X-ray Binaries (LMXBs), with either a black hole or a neutron star (e.g. van der Klis et al. 2004). These quasi-periodic oscillations (QPOs), at frequencies up to 1200 Hz, were discovered by means of the X-ray photon-counters on board of the Rossi X-ray Timing Explorer satellite (RXTE; Bradt et al. 1993). Millisecond time-scales are typical for matter orbiting close to the compact object. Therefore, timing studies of these sources could provide a way to investigate the motion of matter in a strongly curved space-time, thus probing General Relativity in the strong field limit. We describe some ideas on fitting relativistic frequencies in the Kerr metric to observed QPO frequencies in LMXBs. Using a grid of masses and specific angular momenta for the neutron star we show that numerical fits have a low χ2/dof for masses of the neutron star above 2 M⊙. Such masses are bigger than the canonical value 1.4M⊙ measured in double radio pulsars. However, in accreting bynary systems a mass of the neutron star larger than the canonical value has been measured (Casares et al. 2006, 2010). We note that precise measurements of neutron star masses by means of millisecond QPOs are uncertain because of the yet poorly understood phenomenology. If high frequency QPOs in the X-ray flux of LMXBs are produced by orbiting blobs of matter close to the compact object then a full-consistent modelling should also account for the interaction of the shape of the blob with the curved geometry of the space-time. In collaboration with the Department of Mathematics and Physics of the University of Ljubljana we ran simulations of light curves and power spectra produced by clumps of free particles orbiting a Schwarzschild black hole, that are deformed by tidal interaction. The numerical code was developed by (Cadez et al. 2008, Kostic et al. 2009). The numerical simulations reproduce the high frequency part of the power spectrum observed in the black hole LMXB XTE J1550-564 (Germanà et al. 2009).
Le proprietà della variabilità temporale nelle sorgenti astrofisiche sono di notevole interesse e riguardano una vasta gamma di fenomeni che si sviluppano in diversi tipi di oggetti. In questa tesi di dottorato abbiamo investigato due classi di fenomei astrofisici, entrambi legati a studi sulla varabilità temporale. La tesi presenta l'analisi scientifica dei dati raccolti dalla Crab pulsar con gli innovativi contatori di fotoni ottici Aqueye e Iqueye, la cui risoluzione temporale è la più alta mai raggiunta nel dominio ottico (centinaia di picosecondi). Aqueye (Barbieri et al. 2008, 2009) è stato progettato per essere montato al telescopio Copernico in Asiago. Iqueye (Naletto et al. 2009, 2010) è una versione innovativa e progettato per il telescopio NTT in La Silla. Altre investigazioni qui descritte riguardano lo sviluppo e la verifica di idee per interpretare e modellizzare la variabilità temporale al millisecondo osservata in sistemi binari X. Per quanto riguarda l'analisi scientifica dei dati dalla Crab pulsar, essa richiede che ai fotoni raccolti venga associato, con alta precisione, il rispettivo tempo di arrivo secondo un osservatore inerziale. Quindi dobbiamo riferire il tempo di arrivo dei fotoni (TOAs) ad un sistema di riferimento che approssimi al meglio uno inerziale. Solitamente i TOAs all'osservatorio vengono trasformati in TOAs misurati da un osservatore al baricentro del sistema solare. Tempo2 (Hobbs et al. 2006, Edwards et al. 2006) è un software sviluppato per modelizzare con estrema precisione (1 ns) i TOAs misurati in un sistema di riferimento inerziale. Dopo aver baricentrizzato i TOAs, abbiamo usato un codice numerico per calcolare la fase della Crab pulsar. Dallo studio dell'andamento della fase nel tempo è possibile misurare il periodo di rotazione della stella di neutroni e sue derivate. L'analisi dei residui in fase rispetto al modello standard può rivelare peculiarità della sorgente e dell'ambiente circostante. Con questo tipo di analisi e' possibile anche verificare la bontà del modello che corregge i tempi di arrivo al baricentro del sistema solare. Se c'è qualche discrepanza inaspettata allora è interessante investigare sulla sua origine fisica. Un risultato importante ottenuto dall'analisi dei residui in fase è stata la scoperta del primo sistema planetario extrasolare attorno alla pulsar PSR B1257+12 ( Wolszczan & Frail 1992; Wolszczan 1994; Konacki & Wolszczan 2003). Altri studi riguardano verifiche della teoria della Relatività Generale (Helfand et al. 1980; Kramer et al. 2006). Inoltre, il timing delle pulsars è stato proposto come potenziale strumento per la rivelazione di onde gravitazionali (Stappers et al. 2006; Manchester 2010). Dall'analisi dei residui in fase sono state notate inaccuratezze nel ricostruire i TOAs al baricentro del sistema solare, dovute a problemi con i files di configurazione del software Tempo2. Una volta risolti questi problemi, possiamo concludere che i periodi di rotazione della Crab pulsar misurati con Aqueye/Iqueye sono in accordo entro qualche picosecondo con quelli riportati nell'archivio radio del Jodrell Bank Observatory. I TOAs dei fotoni generano una componente di rumore che segue la statistica di Poisson. Le differenze tra i periodi radio e ottici sono maggiori dell'errore Poissoniano stimato. Con i dati raccolti da Aqueye/Iqueye e' stato possibile misurare la derivata prima del periodo di rotazione gia' con osservazioni su una base temporale di soli 2 giorni. Anche in questo caso abbiamo notato discrepanze maggiori dell'errore statistico. Misurando il tempo di arrivo del picco ottico al baricentro del sistema solare e confrontandolo con quello riportato nell'archivio radio, è stato ricavato il ritardo temporale del picco radio rispetto a quello ottico. Il picco ottico arriva circa 120 microsec in anticipo rispetto a quello radio, in accordo con altri osservatori (Shearer et al. 2003; Oosterbroek et al. 2008) . L'analisi ha anche rivelato un deriva della fase ottica rispetto a quella radio che sembra essere legata alle discrepanze gia' menzionate tra i periodi di rotazione. Ulteriori investigazioni hanno portato alla preliminare conclusione che il segnale ottico dalla Crab pulsar potrebbe essere influenzato da una componente di rumore che non segue la statistica di Poisson, conosciuto come timing noise. Rumore non Poissoniano nel segnale da stelle di neutroni è stato rivelato da diversi autori (Boynton et al. 1972; Lyne et al. 1993; Scott et al. 2003; Hobbs et al. 2006b; Patruno et al. 2009), comunque su basi temporali di mesi o anni. Ulteriori osservazioni sono necessarie per verificare la presenza di rumore non Poissoniano su scale di giorni. In questa tesi di dottorato è stata anche esplorata qualche idea sulla interpretazione e modelizzazione della variabilità temporale al millisecondo, osservata nel flusso X delle Low Mass X-ray Binaries (LMXBs; van der Klis 2004). Queste oscillazioni quasi-periodiche (QPOs), a frequenze fino a 1200 Hz, sono state rivelate con i contatori di fotoni X a bordo del satellite Rossi X-ray Timing Explorer (RXTE; Bradt et al. 1993) Oscillazioni al millisecondo sono tipiche del tempo scala orbitale a distanze prossime all'oggetto compatto. Lo studio temporale di queste sorgenti potrebbe essere un modo indiretto per studiare il moto della materia in uno spazio-tempo fortemente curvato, quindi per verificare la teoria della Relatività Generale in regime di campo forte. La tesi descrive qualche idea per interpolare le frequenze dei moti relativistici, calcolate per orbite nella metrica di Kerr, con i QPOs osservati nelle LMXBs. Abbiamo calcolato il chi-quadro ridotto (χ2/dof) su una griglia di masse e momenti angolari e notato che il minimo χ2/dof si ottiene per masse della stella di neutroni maggiori di 2 M⊙. Questi valori sono grandi rispetto alla usuale massa di una stella di neutroni (1.4M⊙) ottenuta dalle pulsar binarie. Comunque, in sistemi binari in accrescimento come le LMXBs, è stata misurata una massa della stella di neutroni maggiore di quella tipica (Casares et al. 2006, 2010). Va precisato che, utilizzare i QPOs al millisecondo per ottenere stime precise della massa di una stella di neutroni potrebbe non essere ancora un metodo sicuro, vista la complessità della fenomenologia e le tuttora poco chiare proprietà. Se i QPOs ad alta frequenza nel flusso X delle LMXBs sono prodotti da corpi che orbitano in prossimità dell'oggetto compatto, allora un modello consistente dovrebbe prendere in cosiderazione anche l'evoluzione della loro forma in uno spazio-tempo curvo. In collaborazione con il Dipartimento di Fisica e di Matematica dell'Università di Ljubljana abbiamo simulato curve di luce e spettri di potenza prodotti da un oggetto costituito da particelle libere orbitanti un buco nero di Schwarschild. Durante il moto orbitale la forma dell'oggetto è fortemente alterata dall'intensa forza mareale del buco nero (Cadez et al. 2008, Kostic et al. 2009). Tali simulazioni numeriche sono in grado di riprodurre lo spettro di potenza osservato nella LMXB con un buco nero XTE J1550-564 (Germanà et al. 2009).

General relativity has been tested by many experiments, which, however, almost exclusively probe weak spacetime curvatures. In this thesis, I create two frameworks for testing general relativity in the strong-field regime with observations of black holes in the electromagnetic spectrum using current or near-future instruments. In the first part, I design tests of the no-hair theorem, which uniquely characterizes the nature of black holes in terms of their masses and spins in general relativity and which states that these compact objects are described by the Kerr metric. I investigate a quasi-Kerr metric and construct a Kerr-like spacetime, both of which contain an independent parameter in addition to mass and spin. If the no-hair theorem is correct, then any deviation from the Kerr metric has to be zero. I show that already moderate changes of the deviation parameters in either metric lead to significant modifications of the observed signals. First, I apply this framework to the imaging of supermassive black holes using very-long baseline interferometry. I show that the shadow of a black hole as well as the shape of a bright and narrow ring surrounding the shadow depend uniquely on its mass, spin, inclination, and the deviation parameter. I argue that the no-hair theorem can be tested with observations of the supermassive black hole Sgr A*. Second, I investigate the potential of quasi-periodic variability observed in both galactic black holes and active galactic nuclei to test the no-hair theorem in two different scenarios. Third, I show that the profiles of relativistically broadened iron lines emitted from the accretion disks of black holes imprint the signatures of deviations from the Kerr metric. In the second part, I devise a method to test the predicted evaporation of black holes in the Randall-Sundrum model of string theory-inspired braneworld gravity through the orbital evolution of black-hole X-ray binaries and obtain constraints on the size of the extra dimension from A0620-00 and XTE J1118+480. I predict the first detection of orbital evolution in a black-hole binary.

Les transitoires à trou noir (BHT) sont parmi les sources X les plus brillantes de la galaxie. Grace à leur flux X intense et leur variabilité rapide, elles offrent une opportunité unique d'étudier la physique de l'accrétion dans des conditions physiques extrêmes. Ces sources présentent des éruptions épisodiques caractérisées par différents niveaux de luminosité en rayons X et gamma, différentes formes spectrales et propriété de variabilité temporelle. Le but de cette thèse est de mieux comprendre la géométrie, les mécanismes et les processus physiques jouant un rôle dans la transitoire X à trou noir brillante GRS 1716-249. Je présente l'analyse spectrale et temporelle des observations de GRS 1716-249 en rayons X effectuées avec l'Observatoire Neil Gehrels Swift durant son éruption de 2016-2017. Ces données m'ont permis d'étudier l'évolution des paramètres physique pendant toute la durée de l'éruption ainsi que les changements de géométrie de la matière accrétante durant les transitions spectrales. Je montre que le disque d'accrétion pourrait avoir atteint la dernière orbite circulaire stable pendant l'état dur intermédiaire. Ceci est en accord avec le modèle de disque tronqué dans lequel bord interne du disque se rapproche de l'objet compact. De plus, le suivi de la source en ondes radio, effectué pendant l'éruption, me permet de placer la source sur la branche radio faible de la correlation des luminosités radio et X. Par la suite, je me concentre sur l'émission en rayons gamma mous de la source. Un excès à haute énergie est détecté au dessus de 200 keV par rapport au spectre de Comptonization thermique. L'origine de cette composante pourrait être l'émission Compton inverse par des électrons non-thermiques dans la couronne, ou l'émission synchrotron d'electron relativistes dans le jet. Je commence par fitter le spectre large bande X/gamma de GRS 1716-249 avec des modèles de Comptonization hybride thermique/non-thermique : eqpair et belm. Les ajustements spectraux avec belm me donnent une limite supérieure sur l'intensité du champs magnétique dans la couronne. Je considère ensuite la possibilité que l'excès à haute énergie soit d au jet. Dans ce but, j'ai produit une distribution spectrale d'énergie s'étendant de la radio jusqu'au rayons gamma. J'ajuste ces données avec un modèle de disque d'accrétion irradié plus Comptonization pour simuler l'émission du flot d'accrétion, et un modèle de shock internes pour l'émission du jet (ishem).[...]
Black hole transients (BHTs) are among the brightest X-ray sources in the Galaxy. Thanks to their high X-ray flux and short variability time scales they offer a unique opportunity to study the physics of the accretion under extraordinary physical conditions. These sources show episodic outbursts characterised by different X/gamma-ray luminosities, spectral shapes and timing properties. The aim of this thesis is the understanding of the geometry, mechanisms and physical processes playing a role in the bright black hole X-ray transient GRS 1716-249. I present the spectral and timing analysis of X-ray observations performed with the Neil Gehrels Swift Observatory on GRS 1716-249 during the 2016-2017 outburst. These data gave me the opportunity to study the evolution of physical parameters and geometry variation of the accreting matter through the spectral transitions during the whole outburst. I found that the accretion disc could have reached the inner stable circular orbit during the hard intermediate state, coherently with the truncated accretion disc scenario in which the disc moves closer to the compact object. Then, the radio monitoring performed during the outburst let me locate the source on the ever more populated radio-quiet branch on the radio/X-ray luminosity plane. Thereafter, focusing on the soft gamma-ray emission of the source, I observed a high energy excess, above 200 keV, in addition to the thermal Comptonisation spectrum. This component could be originate either through inverse Compton of the soft photons by non-thermal electrons in the corona, or from synchrotron emission of energetic electrons in the jet. First, I fitted the broad band X/gamma-ray spectrum of GRS 1716-249 with hybrid Comptonisation thermal/non-thermal models: eqpair and belm. Using belm I obtained an upper limit on the magnetic field intensity in the corona. Finally, I investigated the possible origin of this high energy excess as due to jet emission. To this aim, I computed the Spectral Energy Distribution of GRS 1716-249 with the multi-wavelength observations (from the radio band to gamma-rays) performed. I modelled the accretion flow with an irradiated disc plus Comptonisation model and the jet emission with the internal shock emission model (ishem). This model assumes that the jet velocity fluctuations are directly driven by the variability of X-ray timing proprieties of the accretion flow. Even though ishem reproduces the radio and soft gamma-ray data of GRS 1716-249, the results seems to disfavour the jet scenario for the excess above 200 keV, in favour of non-thermal Comptonisation process

Mes travaux de recherche portent sur l'étude du rayonnement (surtouts rayons X durs) provenant des trous noirs accrétant (dans les noyaux actifs de galaxies et les binaires X). L'objectif est d'en extraire des informations sur les conditions physique régnant dans l'environnement immédiat de ces objets. Les principales question auxquelles je tente de répondre sont les suivantes: Quelle est la structure et la géométrie de la matière accrétée au voisinage du trou noir ? Comment celle -ci évolue-t-elle avec le taux d'accrétion de masse ? Quel est la relation entre les processus d'accrétion et la formation de jets souvent observés dans ces systèmes ? Mon approche est fondée sur une comparaison précise entre les observations et les prédictions des divers modèles. Je présente les efforts poursuivis depuis près de dix ans afin de développer des outils de simulation numérique pour modéliser le transfert de rayonnement dans les plasma chauds des sources compactes X. Je montre comment ces outils ont été utilisés pour modéliser le continuum haute énergie et la variabilité des trous noirs accrétants et pour contraindre la structure du flot d'accrétion. Je présente également des résultats reposant sur l'analyse et l'interprétation d'observations menées avec des télescopes spatiaux tels que XMM-Newton et INTEGRAL ainsi que sur des d'observations simultanées à plusieurs longueurs d'ondes allant de la radio aux rayons X durs.

Galactic black hole X-ray transients (BHXRTs) represent the ideal opportunity to study accretion physics in extreme environments. Questions relating to accretion geometry, the mass of the compact object, how transient outbursts start and how jets form and evolve can only be answered by observing BHXRTs across multiple wavelengths. In this thesis I take a multi-wavelength approach to studying BHXRTs, and attempt to reconcile many aspects of accretion physicsusing a number of techniques. I have used timing analysis techniques to discover a super-orbital periodicity of 420 days in the X-ray and optical light curves of the BHXRT Swift J1753.5-0127, whilst also uncovering the likely orbital period of the candidate BHXRT MAXI J1305-704. X-ray spectral observations of Swift J1753.5-0127 revealed its first ever transition to a soft accretion state. This was found to be one of the lowest luminosity soft states ever recorded in such a system, at < 1% the Eddingtonluminosity, which proved crucial in the subsequent radio observations. Collaborators and I found that the compact jet had been quenched by a factor > 25, indicating that jet quenching was not dependent on accretion rate. In the optical regime, spectroscopy can reveal details about the mass of the compact object, which is important for population studies of black holes (BHs). I place a lower limit on the mass of Swift J1753.5-0127at M1 > 7:41:2M, conrming it as a BH and removing it from the so-called 'mass gap'. However, optical observations can also reveal details about the accretion geometryof a source, as in the case of the BHXRT V404 Cyg, in which collaborators and I combined optical spectroscopy and photometry and found evidence for the 2015 outburst initiating 1 week before the X-ray outburst was detected. Our results were found to be consistent with the current disc instability model for transient outbursts. I summarise these ndings and suggest avenues for future work in the closing pages of this thesis. Overall, this work shows that we can only truly understand BHXRTs once we have studied them across multiple wavelengths, and only in doing so can we discuss the underlying physics behind some of the most extreme regions of the Universe.

Remarkably, an astrophysical black hole has only two attributes: its mass and its spin angular momentum. Spin is often associated with the exotic behavior that black holes manifest such as the production of relativistic and energetic jets. In this thesis, we advance one of the two primary methods of measuring black hole spin, namely, the continuum-fitting method by (1) improving the methodology; (2) testing two foundational assumptions; and (3) measuring the spins of two stellar-mass black holes in X-ray binary systems. Methodology: We present an empirical model of Comptonization that self-consistently generates a hard power-law component by upscattering thermal accretion disk photons as they traverse a hot corona. We show that this model enables reliable measurements of spin for far more X-ray spectral data and for more sources than previously thought possible. Testing the foundations: First, by an exhaustive study of the X-ray spectra of LMC X–3, we show that the inner radius of its accretion disk is constant over decades and unaffected by source variability. Identifying this fixed inner radius with the radius of the innermost stable circular orbit in general relativity, our findings establish a firm foundation for the measurement of black hole spin. Secondly, we test the customary assumption that the inclination angles of the black-hole’s spin axis and the binary’s orbital axis are the same; for XTE J1550–564 we show that they are aligned to within \(12^{\circ}\) by modeling the kinematics of the large-scale jets of this microquasar. Measuring spins: We have made the first accurate continuum-fitting spin measurements of the black hole primaries in H1743–322 and XTE J1550–564. For this latter black hole, we have also measured its spin using the other leading method, namely, modeling the broad red wing of the \(Fe K\alpha\) line. As we show, these two independent measurements of spin are in agreement.
Astronomy

The micro Seyfert galaxy NGC-4395 hosts a supermassive black hole at its center that actively accrete matter and thus radiates very strongly in the X-ray range. Denoted the active galactic nucleus this thesis focuses on determining its spin. The spin parameter corresponds to the angular momentum of the black hole and the properties of the accretion disk surrounding the black hole. Our spectrum was obtained from satellite telescope XMM-Newton and was analysed in the range of 3 to 10 keV using the X-ray spectral fitting software XSPEC with the standard models provided. A good fit was found, and therefore a good description of the parameters describing the black hole and its disk. These start values were used for the far more advance model KERRCONV*REFLIONX which contains the spin. Specially noteworthy is the broad and narrow Iron Kemission lines normally occurring at 6:4 keV whose form tells which different relativistic or gravitational effects that affects the emissivity. In our spectrum both the broad and narrow Fe emission lines were prominent as shown by the significant improvement in 2. Furthermore the reflected component of our fit completely dominated our spectra and a majority of the emission originates from the innermost regions of the disk, probably caused by strong gravitational light bending revealing a closely positioned corona. The high value obtained on the spin parameter seems to be consistent with the relatively low mass and possibly implies a coherent accretion during the black holes formation.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.
Includes bibliographical references (p. 93-94).
GRS 1915+105 is an accreting black-hole in a binary system located in the Milky Way. It is one of the most variable X-ray sources known, and 12 variability classifications have been defined, many of which appear to be repetitive cycles of accretion instability. We study one particular variability type, the p cycle, which is selected for its high frequency quasi-periodic oscillations (HFQPOs) and recurring double-peak flare in the light curve. We investigate the primary properties of the 82 p-type observations collected by RXTE. The range in flare recurrence time () is 33.73 s < T < 122.49 s, with <> ± asample = 65.44 ± 19.83 s. The flaring fraction , defined by percent of cycle exposure > 1.2*mean count rate, ranges 12.11% < ( < 37.61%, with <> ± asample = 20.05 ± 5.33%. We find a correlation between T and ( which divides the 82 observations into three sub-classes: pi; slow with low , P2; fast with low , and P3; fast with high . The evolution between sub-classes suggests two driving mechanisms, an unknown mechanism limiting T > 33 s and a process consistent with the Eddington limit that increases (at the lower limit of ) for the p3 group.
(cont.) For each subclass we study the emission properties in four phase zones of the p cycle, where the phases are defined on the basis of the X-ray count rate (X) and soft color (S; rates at 6-12 keV / 2-5 keV). Two HFQPOs in the p cycle are isolated to different zones and sub-classes: one at 67 Hz is localized to the second (hard-spectrum) flare, and another QPO at 150 Hz in the low X, low S phase zone of the pi group. All phase zones display low-frequency QPOs, and they are particularly strong in the low-X, low-S zone (7.5 Hz) and the low-X, high-S zone (10.5 Hz). Classifications of X-ray spectral states for each zone indicate no zones in the thermal state, flaring zones (high X) in the steep power law (SPL) state, and quiet zones (low X) in either the hard or hard:SPL intermediate state. We conclude that the p cycle provides special opportunities to further study an instability cycle that is driven, in part, by the Eddington limit and that portions of the cycle contain the mechanism that produces two different HFQPOs. Further investigations should be made with increased phase resolution and with additional strategies to define the phases of the p cycle.
by Jonathan Zachary Gazak.
S.B.

Ultraluminous X-ray sources (ULXs) are a class of extragalactic, off nuclear and point-like sources with isotropic X-ray luminosities higher than 1e39 erg/s. They are supposed to be accreting Black Hole binaries systems but the accretion mechanisms at the basis of their extremely high X-ray luminosity are still matter of debate. We carried out a detailed spectral analysis of all the available XMM-Newton observations of two ULXs in NGC 1313, adopting a common model based on a multicolor disc plus a comptonizing component. We were able to describe the spectral evolution of the two sources within such a common framework. Furthermore, we investigated the chemical abundances of their local environments making use of both EPIC and RGS data. The results appear to indicate sub-solar metallicity for both sources. The possible existence of two spectral states in NGC 1313 X-1 and X-2 suggested to look for similar behaviours also in other ULXs. We then studied a larger sample of sources, including IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1, NGC 55 ULX1 and NGC 253 X-1. These sources were selected because they have a luminosity higher than 2e39 erg/s, are nearby, have one long observation and at least three other observations. The high quality observations provide at least 10000 counts in the EPIC instruments allowing us to constrain the curvature at high energy and to perform an analysis of the abundances of the material along the line of sight. We found that, in most of the spectra of the sources of our sample, the high energy component has a low temperature and is optically thick. However, because of the poor quality of some observations, the spectral fits are sometimes affected by a degeneracy between the spectral parameters and the roll-over of the spectrum at high energy is not easy to detect. For these reasons, similarly to what has been done for low counting statistics spectra of Galactic X-ray binaries (XRBs), we adopted the method of the hardness ratios that has also the advantage to allow us to study the spectral variability in a way completely independent of the spectral models. This analysis suggests the existence of possible characteristic evolutionary patterns on the color-color and intensity-color diagrams linking at least two different spectral states. This behaviour can be explained in terms of a non-standard accretion disc in which the increment of the accretion rate produces outflows that become more and more important at the highest luminosities. We tested the scenario of the ejection of a wind jointly analyzing the spectral and timing properties of the source NGC 55 ULX1 which shows a puzzling flux variability. In fact, fast drops in the flux are observed on time scales of minutes to hours that may be produced by optically thick blobs of matter that from time to time encounter our line of sight. We compared its variability properties with those of a Galactic accreting systems, EXO 0748-676, which is powered by a neutron star and is a known dipping source. We characterised the nature of the variability observed in the power density spectrum and, in particular, we checked the presence of a linear relation between the Root Mean Square (RMS) variability and the flux in several energy bands. We found that, in EXO 0748-676, the predominance of an (ionised) absorber strongly affects the RMS-flux relation which may anticorrelate when the absorption lines are unsaturated. On the other hand, no further variability is introduced when they are saturated and the variability is dominated by the accretion flow. In this case the source shows a positive correlation between RMS and flux. Since we found an anticorrelation in NGC ULX1, we suggest that at the highest flux levels, massive and unsaturated turbulent outflows are ejected. Finally, persistent ULXs, as those discussed above, do not allow us an easy comparison with the behaviour of Galactic XRBs. Transient ULXs are much more promising in this respect as they span different accretion regimes. Till now, only a handful of transient ULXs has been discovered and the link between them and the persistent sources is still unclear. We monitored the evolution of a new ULX (XMMU J004243.6+41251) discovered in January, 2012 in M31 by XMM-Newton. Its outburst showed that, at maximum luminosity, it entered in the ULX regime. It was then extensively followed by Swift during the flux decay. The source has experienced a fast rise in flux after discovery during which the XMM-Newton spectra changed from a powerlaw-like to a disk-like shape in the Swift spectra, suggesting a transition between the canonical low/hard and high/soft states. Its luminosity remained fairly constant for at least 40 days and then it faded below 1e38 erg/s. During the decay the disc emission softened and the temperature decreased from ~0.9 keV to ~0.5 keV. An optical follow-up and the UVOT images failed to provide evidence of a counterpart down to 22 mag in the optical band and to 23-24 mag in the near Ultraviolet. We compared the properties of XMMU J004243.6+412519 with those of other known ULXs and Galactic black hole transients, finding more similarities with the latter.
Le Ultraluminous X-ray sources (ULXs) sono una classe di sorgenti extragalattiche, lontane dal nucleo della galassia ospite e puntiformi, con una luminosità isotropica maggiore di 1e39 erg/s. Si pensa siano buchi neri in accrescimento in sistemi binari ma i meccanismi di accrescimento alla base della loro estrema luminosità X sono ancora lontani dall'essere totalmente compresi. In questo lavoro è stata svolta una dettagliata analisi spettrale di tutte le osservazioni disponibili di XMM-Newton di due ULXs in NGC 1313, adottando un modello comune basato su un disco multicolore più una componente di comptonizzazione. Noi siamo stati capaci di descrivere l'evoluzione spettrale delle due sorgenti all'interno di tale scenario. Inoltre, è stato possibile determinare le abbondanze chimiche dei loro ambienti locali facendo uso sia di dati EPIC che di dati RGS. I risultati sembrano indicare metallicità sub-solare per entrambe le sorgenti. La possibile esistenza di due stati spettrali in NGC 1313 X-1 e X-1 hanno suggerito di cercare comportamenti simili anche in altre ULXs. Per questo motivo, un campione più vasto di sorgenti, il quale include IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II X-1, NGC 55 ULX1 e NGC 253 X-1, è stato studiato. Queste sorgenti sono state selezionate poichè la loro luminosità è maggiore di 2e39 erg/s, sono sorgenti vicine e posseggono un'osservazione lunga e almeno altre tre ulteriori osservazioni. L'alta qualità delle osservazioni fornisce almeno 10000 conteggi nello strumento EPIC, consentendoci di determinare con più precisione la curvatura ad alta energia e di svolgere un'analisi delle abbondanze del materiale presente lungo la linea di vista. E' stato trovato che, nella maggior parte degli spettri delle sorgenti del nostro campione, la componente ad alta energia mostra una bassa temperatura ed è otticamente spessa. Ad ogni modo, a causa della bassa qualità di alcune osservazioni, i fit spettrali sono a volte influenzati da una degenerazione fra i parametri spettrali e la curvatura ad alta energia dello spettro non è facilmente individuata. Per queste ragioni, in modo simile a ciò che è stato ampiamente fatto per gli spettri di sorgenti binarie Galattiche di raggi X, abbiamo adottato il metodo degli hardness ratios che hanno anche il vantaggio di consentirci di studiare la variabilità spettrale in un modo completamente indipendente dal modello spettrale. Questa analisi suggerisce l'esistenza di un possibile caratteristico cammino evolutivo sui diagrammi colore-colore and intensità-colore collegando almeno due differenti stati spettrali. Questo comportamento può essere spiegato in termini di un disco di accrescimento non standard in cui l'aumento del tasso di accrescimento produce fuoriuscite di materiale che diventano via via più importanti alle più alte luminosità. Lo scenario di emissione di vento è stato ulteriormente studiato analizzando le proprietà spettrali e temporali della sorgente NGC 55 ULX1 che mostra un'enigmatica variabilità nel flusso. Infatti, rapide diminuzioni del flusso emesso sono osservate su tempi scala di minuti od ore che potrebbero essere prodotti da nuvole di materiale otticamente spesso che di tanto in tanto entrano all'interno della nostra linea di vista, oscurando le regioni centrali della sorgente. E' stata fatta un'analisi comparativa fra le proprietà della sua variabilità con quelle di un sistema Galattico in accrescimento, EXO 0748-676, conosciuto per ospitare una stella di neutroni e per essere una sorgente con “dips". Abbiamo caratterizzato la natura della variabilità osservata negli spettri di potenza e, in particolare, abbiamo testato la presenza di una relazione lineare tra la variabilità quadratica media (RMS) e il flusso in diverse bande di energia. E' stato trovato che, in EXO 0748-676, la predominanza di un mezzo assorbente (ionizzato) influenza fortemente la relazione fra RMS e flusso che potrebbero anti-correlare qualora le linee in assorbimento fossero non sature. D'altra parte, nessuna variabilità ulteriore è introdotta quando esse sono sature e la variabilità è dominata dal flusso d'accrescimento. In questo caso la sorgente mostra una correlazione positiva tra flusso ed RMS. Poichè noi abbiamo individuato un'anti-correlazione in NGC 55 ULX1, proponiamo che ai livelli di flusso più alti, imponenti venti, non saturi e turbolenti, siano eiettati. Infine, ULX persistenti come quelle discusse sopra, non consentono una facile comparazione con il comportameno delle sorgenti binarie Galattiche. ULX transienti sono molto più promettenti sotto questo punto di vista poichè esse attraversano differenti regimi di accrescimento. Fino ad ora, solo una manciata di ULX transienti sono state scoperte e la connessione tra loro e le sorgenti persistenti è ancora poco chiara. Noi abbiamo monitorato l'evoluzione di una nuova ULX (XMMU J004243.6+41251) scoperta nel Gennaio 2012 nella galassia M31 da XMM-Newton. La sua accensione ha mostrato che, alla luminosità di picco, la sorgente è entrata nel regime ULX. E' stata poi ampiamente seguita da Swift durante la sua fase di decadimento in flusso. La sorgente ha sperimentato un veloce incremento del flusso dopo la sua scoperta, durante il quale gli spettri ottenuti da XMM-Newton si sono evoluti da un semplice andamento a legge di potenza fino ad una forma tipica per un disco d'accrescimento in tutti gli spettri Swift, suggerendo una transizione tra gli stati canonici low/hard and high/soft. La sua luminosità è rimasta abbastanza costante per almeno 40 giorni, per poi scendere al di sotto di 1e38 erg/s. Durante il decadimento, l'emissione del disco è diventata più soft e la temperatura è diminuita da ~0.9 keV fino a ~0.5 keV. Un follow-up ottico e immagini UVOT non sono riuscite a fornire evidenze di una controparte fino a 22 mag in banda ottica e fino a 23-24 mag nel vicino Ultravioletto. Noi abbiamo comparato le proprietà di XMMU J004243.6+41251 con quelle di altre ULX transienti e buchi neri Galattici, trovando più similitudini con le ultime.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2005.
Includes bibliographical references (p. 61-62).
In this thesis, we describe the analysis of 0.1-4 Hz quasi-periodic oscillations (QPOs) of the black hole X-ray binary XTE J1550-564 using data obtained with the Rossi X-ray Timing Explorer (RXTE) satellite. Data from 13 consecutive observations spanning eight days were included in this analysis. By comparing the spectra of the source when it is in the high intensity phase with the low intensity phase of its QPO cycle, we hoped to gain insight into the cause of QPOs and how they relate to structures and processes in the vicinity of black holes. Like observations from the black hole X-ray binary GRS 1915+105 (Miller and Homan 2005), our observations from XTE J1550-564 showed a significant difference in the Fe ... line equivalent width between the high and low intensities of the QPO for 6 of 13 observations. However, 2 of 13 observations exhibited the opposite effect in that the Fe K [alpha] line equivalent width actually dropped significantly during periods of high intensity. Moreover, we found that the integrated flux of the Fe K [alpha] line was proportional to the flux of the continuum in 5 of 13 observations. Despite these competing effects, the ratios of the low intensity and high intensity spectra indicate an increase in the QPO strength up to about 10 keV, above which it decreases only slightly. We also found dramatic changes in the QPO strength at low energies as the QPO frequency increases. We believe this to be the result of an increase in relative contributions to the spectra as the disk blackbody component becomes more important.
(Cont.) In this thesis, we discuss five possible spectral models to explain QPOs based on the behavior of Fe K [alpha] emission lines. Then we describe whether or not our results agree with those predictions. Although our results show correlations similar to those found by Miller and Homan (2005), they also show anti-correlations in several observations. Our results support Miller and Homan's suggestion of a link between discrete timing features (QPOs) and spectral features (Fe K [alpha] emission lines) which occur in the inner disk around black holes. Our work shows that current explanations for QPOs have some physical basis, suggesting that QPOs are due to either a quasi-periodically changing reflector area in the accretion disk or an episodic modulation of the hard flux component in the corona or jets that irradiates the accretion disk. Our results also suggest that perhaps the correct model is something more complex that goes beyond current models and is able to explain the multiple effects observed in the Fe K [alpha] line.
by Steven N. Cho.
S.M.

Significant X-ray variability and flaring has been observed from Sgr A* but is poorly understood from a theoretical standpoint. We perform general relativistic magnetohydrodynamic simulations that take into account a population of non-thermal electrons with energy distributions and injection rates that are motivated by PIC simulations of magnetic reconnection. We explore the effects of including these non-thermal electrons on the predicted broadband variability of Sgr A* and find that X-ray variability is a generic result of localizing non-thermal electrons to highly magnetized regions, where particles are likely to be accelerated via magnetic reconnection. The proximity of these high-field regions to the event horizon forms a natural connection between IR and X-ray variability and accounts for the rapid timescales associated with the X-ray flares. The qualitative nature of this variability is consistent with observations, producing X-ray flares that are always coincident with IR flares, but not vice versa, i.e., there are a number of IR flares without X-ray counterparts.

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.
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.

Comme la plupart des galaxies massives, la Voie lactée héberge en son centre un trou noir supermassif. Contrairement aux noyaux actifs, celui-ci brille par la faiblesse de sa contrepartie radiative, la source Sgr A?, dont la luminosité est inférieure de près de neuf ordres de grandeur à la luminosité limite d’Eddington. Plusieurs traces indirectes suggèrent cependant que le trou noir galactique a connu des épisodes d’activité plus intense dans le passé. La réflexion des rayons X émis lors des plus récents de ces épisodes est notamment observée aujourd’hui sur les nuages de la zone moléculaire centrale. À partir de l’observation de ces échos de lumière, cette étude offre une vision unique de l’activité du noyau galactique au cours du dernier millénaire. Elle confirme en particulier que l’émission non thermique diffuse du complexe moléculaire Sagittarius C, étudiée avec les satellites Chandra et XMM-Newton, résulte bien du phénomène de réflexion. La localisation individuelle des structures brillantes est rendue possible par l’utilisation de modèles spectraux Monte-Carlo, dont le modèle rayflX développé spécifiquement pour rendre compte du phénomène de réflexion et, notamment, de ses aspects temporels. À partir des spectres de huit nuages issus des quatre principaux complexes moléculaires des régions centrales de la Galaxie, ce travail fournit la première preuve quantitative avec un haut niveau de confiance de l’existence de deux sursauts d’activité de Sgr A?se propageant dans la zone moléculaire centrale, dont l’âge, la durée et la luminosité peuvent être déterminés. Ces résultats fournissent de nouvelles contraintes pour l’étude du cycle d’activité de Sgr A? et ouvrent la voie à une caractérisation par tomographie de la distribution de matière au centre de notre galaxie
Like most massive galaxies, the Milky Way hosts a supermassive black hole at its centre. In contrast with active galaxies, the luminosity of its radiative counterpart — the source Sagittarius A* — is almost nine orders of magnitude below the Eddington limit. However, there are several indications that the black hole may have been more active in the past. In particular, the giant clouds located within the central molecular zone reflect X-rays emitted during the most recent outbursts. The study we present here focuses on these light echoes that are key to probing the past activity of the Galactic nucleus over the past thousand years. Using the X-ray observatories Chandra and XMM-Newton, we confirm the reflection origin of the non-thermal, diffuse emission from the molecular complex Sagittarius C. The localisation of each bright clump is made possible by the use of Monte Carlo-simulated reflection spectra, notably the rayflX model we specifically designed for the study of the reflection phenomenon and its time behaviour. Considering eight clouds from the main molecular complexes of the Galactic central regions, this work provides with a high confidence level the first quantitative evidence that Sgr A* experienced two powerful outbursts, whose age, duration and luminosity can be estimated. These results place new constraints on the black hole duty cycle and pave the way for a tomographic reconstruction of the matter distribution at the Galactic centre

In this thesis I investigate the population of X-ray binaries (XRB) and ultraluminous X-ray sources (ULX) in order to approach several contemporary astrophysical conundrums. My main method of analysis is the population synthesis, which allows for a comprehensive comparison of theoretical models and observations. I provide an introduction with fundamental knowledge concerning XRBs and ULXs. Firstly, I approach a problem of the mass gap, i.e., the lack of compact objects with masses between 2 5 M . I show that the rapid supernova explosion mechanism can provide a natural explanation for the observed separation between neutron stars and black holes. Afterwards, I investigate the common envelope. This important phase of evolution of binaries still escapes the grasp of our understanding. I show that, although the common envelope phase is essential for the formation of XRB, none of currently available models is able to reproduce the observations. I analyse also the formation of the most luminous ULXs. I show that it is possible to obtain mass transfer rates high enough to power such a source in regular XRBs. The phase of powerful emission will be very short but present in the evolution of numerous systems. Finally, I present the preliminary results of the investigation of accretion models in the context of the ULX population. I compare the synthetic population, which is based on our best knowledge of astrophysical processes, with the observations in order to understand the nature and formation processes of these systems.

博士
國立中央大學
天文研究所
106
Low-frequency quasi-periodic oscillations (LFQPOs) with frequencies ranging from a few millihertz to 30 Hz are non-stationary astrophysical phenomena observed in most of black hole X-ray binaries (BHXBs). According to their Fourier power spectral shapes and fitting parameters, LFQPOs are further classified into A, B and C types. Previous time-frequency analysis research showed that LFQPOs are composed of frequency varying oscillations appearing occasionally. However, due to the time-frequency limitation and prior mathematical assumptions (a constant frequency or a waveform), the time-frequency analysis methods based on Fourier or wavelet transforms are difficult to resolve further information beyond the limitation and their assumptions may be too strict to be consistent with the properties of LFQPOs. Therefore, I adopted a recently developed time-frequency analysis method, the Hilbert-Huang transform (HHT), to cross the limitations for analyzing LFQPOs. HHT can instantaneously track phase, frequency, and amplitude variations of non-stationary signals such as LFQPOs, without strictly mathematical assumptions regarding the oscillatory components. To track detailed frequency and amplitude variations of LFQPOs, we first apply HHT on a 4-Hz type-C LFQPO from the BHXB XTE J1550-564. By adaptively decomposing the ∼ 4-Hz oscillatory component from the X-ray light curve and acquiring its instantaneous frequency, the Hilbert spectrum illustrates that the LFQPO is composed of a series of intermittent oscillations appearing occasionally between 3 Hz and 5 Hz. We further characterized this intermittency by computing the confidence limits of the instantaneous amplitudes of the intermittent oscillations, and constructed both the distributions of the QPO’s high and low amplitude durations, which are the time intervals with and without significant ∼ 4-Hz oscillations, respectively. The mean high amplitude duration is 1.45 s and 90% of the oscillation segments have lifetimes below 3.1 s. The mean low amplitude duration is 0.42 s and 90% of these segments are shorter than 0.73 s. In addition, these intermittent oscillations exhibit a correlation between the oscillation’s rms amplitude and mean count rate. This correlation could be analogous to the linear rms-flux relation found in the 4-Hz LFQPO through Fourier analysis. We conclude that the LFQPO peak in the power spectrum is broadened owing to intermittent oscillations with varying frequencies, which could be explained by using the Lense-Thirring precession model. Based on the successful application to the 4-Hz type-C LFQPO around XTE J1550-564, we further utilized HHT to track X-ray spectral modulations of 14 type-B LFQPOs in the BHXB GX 339-4. It has been shown that type-B QPO frequencies have strong correlation with the hard X-ray flux, but the detailed variations of hard X-ray spectral components during the oscillation are still not clear. To track modulations of spectral parameters, we utilized the HHT to characterize the HHT-based timing properties, extract the QPO instantaneous phases, and then construct its phase-resolved spectra. We found that these QPOs are composed of a series of intermittent oscillations with coherence times less than ∼ 1 s. Furthermore, the phase-resolved spectra illustrate significant modulations of Comptonization parameters with much smaller but also significant modulation of thermal disk component. We discussed possible interpretations of the spectral modulations. Finally, I summarized my research works and pointed out possible future applications of the HHT on LFQPOs.

We study the spectro-temporal properties of the black hole X-ray binaries (BHXBs) GRS 1915+105 and IGR J17091-3624 with data from RXTE, Chandra and XMM-Newton. From the analysis result, we propose models of accretion modes in terms of known accretion classes that might be responsible for the different correlated spectral states with temporal classes, namely: Keplerian disc flow (Shakura & Sunyaev, 1973a), slim disc flow (Abramowicz et al., 1988), advection dominated accretion flow or ADAF (Narayan & Yi, 1994) and general advective accretion flow or GAAF (Rajesh & Mukhopadhyay, 2010). We further posit that accretion rate must play an important role in transition between these states. While GRS 1915+105 exhibits all four modes, in IGR J17091-3624, only two of the modes – ADAF and slim disc – are evident, although the effect of Poisson noise cannot be ruled out. Thus, we infer that while both sources show a lot of similarities in their lightcurves, the underlying nonlinear dynamical properties of their accretion flows may be different. With long, continuous optical/UV and X-ray data from AGNs, 134 Summary and future prospects this model can be tested, as any consistent result will throw more light on how the mechanisms at work in BHXBs and AGNs are related. The energy spectra of the Seyfert 1 AGN Zw 229.015 reveals the presence of strong soft X-ray emission below 1keV in excess of the primary X-ray power-law. In an attempt to probe the plausible origin of the soft excess, we apply four different models, namely: multicolour disc blackbody (Mitsuda et al., 1984; Makishima et al., 1986), smeared wind absorption (Gierli´nski & Done, 2004), thermal Comptonisation (Titarchuk, 1994) and relativistically blurred reflection (Ross & Fabian, 2005) models. We find that both thermal Comptonisation and relativistically blurred reflection models provide acceptable explanation to the origin of the soft X-ray excess. Further, based on soft/hard X-ray time lag, we constrain the size of its corona to be ∼ 20Rg – a value which is consistent with earlier works. Through cross-correlation analysis of the UV/X-ray lightcurves of the Seyfert 1 AGN Mrk 493, we measure a lag of 5ks in which the UV emission lead the X-rays in their variability. To explain this lag, we calculate different variability timescales associated with accretion discs. From our estimations, the measured lag is consistent with the time required for UV photons produced in the disc to travel to the hot corona and the time required for repeated inverse Compton scattering in the corona. This implies that thermal Comptonisation of disc UV emission in the corona is responsible for the observed X-rays. Although this scenario has been predicted theoretically, we have been able to confirm it observationally for the first time. Therefore, overall: (1) we have shown through correlated spectral and nonlinear time series analysis that accretion flows around compact object – black holes – could reveal deterministic or stochastic behaviour over time with implications for modes of accretion. (2) Through arguments from cross-correlation analysis and energy spectral properties, we put constraints on corona geometry and show its emission mechanism

A system of two stars in which one is a black hole (BH) is referred to as a black hole binary (BHB). In a BHB, it is possible to have mass transfer from the secondary star (companion star) to the primary black hole. Matter from the secondary star is transferred to the BH by accretion, which is a highly efficient process in converting rest mass energy into radiation. BHB systems emit significant radiation, mainly in X-rays and has helped astrophysicists and astronomers to study the physical processes leading to this emission. Several X-ray astronomy missions have been commissioned in the last few decades to observe such sources. The BH X-ray binaries (XRBs) usually have phases of significant X-ray activity known as outbursts in between quiescent states. We use data from several X-ray observatories to carry out spectro-temporal analysis, modelling of the data and interpretation of the source characteristics during the outburst phase of Galactic Black Holes (GBHs).

Black hole (BH) accretions are very powerful sources of energy in the universe. In accretion phenomena, the surrounding gas spirals down towards the central BH and forms a disc or quasi-spherical structure based on the outward transport of angular momentum. At the same time, the enormous gravitational potential energy of a BH is released as heat and radiation. However, the presence of strong magnetic fields can alter the underlying radiation mechanisms and thermodynamic properties. Theoretical and observational inferences indicate a signature of dynamically dominant magnetic fields in the vicinity of BHs. It was suggested that the externally generated large-scale magnetic fields could be captured from the environment, say, interstellar medium or companion star, and dragged inward by the accreting plasma. This magnetic field is significantly compressed and becomes dynamically dominant through flux freezing due to continued inward advection of the magnetic flux in the quasi-spherical accretion flow near the BH. This thesis work is on understanding the role of large-scale strong magnetic fields on optically thin and advective accretion flows in which magnetic fields influence the accretion dynamics, remove angular momentum from in-falling matter, help in the formation of strong outflows, and enhance the cooling mechanism through synchrotron and synchrotron self-Comptonization processes. First, we discuss the importance of large-scale strong magnetic fields in the removal of angular momentum outward, as well as the possible origin of different kinds of magnetic barrier in optically thin, geometrically thick, sub-Keplerian, advective accretion flows around BHs. To treat the full set of magnetohydrodynamics equations, we solve the magnetic fields self-consistently along with the gas dynamics in the general advective paradigm. Here, we consider the pseudo-Newtonian framework with Paczyński \& Witta potential to mimic the space-time geometry around non-rotating BHs. In this simplest vertically averaged, 1.5-dimensional disc model, we choose the maximum upper limit of the magnetic field, which the disc around a BH can sustain. We have shown that in our Magnetically Arrested Advective Accretion Flow (MA-AAF) model, the accreting gas either decelerates or faces the magnetic barrier near the event horizon by the accumulated magnetic fields depending on its geometry. The magnetic barrier may knock the matter to infinity. We suggest that these types of flow are the building block to produce strong jets and outflows in the accreting system. We also find that in some cases, when the matter is trying to go back to infinity after knocking the barrier, the matter is prevented from being escaped by the cumulative action of strong gravity and the magnetic tension, hence by another barrier. In this way, the magnetic field can lock the matter in between these two barriers, and it might be a possible explanation for the formation of episodic jets. Next, we construct a coupled disc-outflow model around BHs in the MA-AAF paradigm. As an immediate observational consequence, we have applied our disc-outflow symbiosis to explain some long-standing issues of ultraluminous X-ray sources (ULXs), which are very bright, off-nuclear, point sources with luminosity exceeding the standard Eddington limit for a stellar-mass BH. The existing physical scenarios to explain their unusual high luminosity and spectral nature are either the existence of the missing class of intermediate-mass BH (IMBH) or super-Eddington accretion around a stellar-mass BH. However, most ULXs with a steep power-law spectrum can be well explained through super-Eddington accretion, while the IMBH scenario has been disputed extensively. Nevertheless, the interpretation of ULXs with a hard power-law-dominated state remains mysterious. Here we show that the flow energetics of a 2.5-dimensional advective magnetized accretion disc-outflow system around a stellar-mass BH are sufficient to explain the power of ULXs in their hard states. Hence, there is neither need to incorporate the contentious IMBH scenarios nor super-Eddington accretions. We suggest that at least some ULXs are magnetically powered sub-Eddington accretors around a stellar-mass BH. We further extend our disc-outflow symbiotic model to the general advective, two-temperature framework in the MA-AAF paradigm with explicit cooling. Here, we include the effects of magnetic fields, gas and radiation counterpart together in the entropy gradient based on the first law of thermodynamics to represent energy advection for ions and electrons separately, for the first time. The cooling process includes bremsstrahlung, synchrotron radiation, and inverse-Comptonization processes. Here, we consider the pseudo-Newtonian framework with Mukhopadhyay potential to mimic the space-time geometry around rotating BHs. Interestingly, in our optically thin MA-AAF solutions, the advection of both poloidal and toroidal magnetic fields is happening, unlike other magnetically dominated accretion models. Also, unlike the previous exploration in the framework of disc-outflow symbiosis, the dynamics here is primarily controlled by large-scale magnetic stress. The main objective here is to explain the bright, hard-state observations of accreting systems with stellar-mass to supermassive BHs. One of our main ventures is to find out the hidden nature of mysterious hard-state ULXs. Most importantly, the power-law photon index of these ULXs persists despite their X-ray luminosity varying by over an order of magnitude. Also, these ULXs are very rare in nature. We have shown that our magnetically dominated disc-outflow symbiosis around rapidly spinning stellar-mass BHs can achieve such large luminosity even for the sub-Eddington accretion rate. The magnetic field at the outer zone of the advective flow is more than the corresponding Eddington limit. Such a field becomes dynamically dominant near the BH through continuous accretion process due to flux freezing but maintaining its Eddington limit. This unique field configuration enhances the synchrotron and synchrotron self-Comptonization processes to achieve very large luminosity. However, such magnetic fields are not always possible to capture from the environment, say, companion stars, thus explaining such ULXs to be rare. Our solutions for supermassive BHs can explain the unusual large luminosity of ultra-luminous quasars through the same mechanism. Finally, we have addressed a unified classification of blazars, a particular class of active galactic nucleus (AGN), by solving magnetized disc-outflow symbiosis self-consistently and compared with $Fermi$ blazar observations. Blazars are radio-loud AGNs with relativistic jets pointing close to our line of sight. Based on the equivalent width (EW) of the optical emission lines, blazars are classified into two subclasses: flat-spectrum radio quasars (FSRQs) with EW $\geq 5$ {\AA} and BL Lac objects with EW $<5$ {\AA}. The signature of strong emission lines suggests an efficient accretion process in FSRQs compared to BL Lacs. According to the $Fermi$ blazar observations, BL Lac objects are less luminous with harder spectra than FSRQs. We compute the jet intrinsic luminosities by beaming corrections determined by different cooling mechanisms. Observed $\gamma$-ray luminosities and spectroscopic measurements of broad emission lines suggest a correlation of the accretion disc luminosity with the jet intrinsic luminosity. Also, theoretical and observational inferences for these jetted sources indicate a signature of hot advective accretion flow and a dynamically dominant magnetic field at the jet footprint. Indeed it is difficult to imagine the powerful jet launching from a geometrically thin Keplerian disc. We propose a magnetized, advective disc-outflow symbiosis with explicit cooling to address a unified classification of blazars by controlling both the mass accretion rate and magnetic field strength. We suggest that the BL Lacs are more optically thin and magnetically dominated than FSRQs at the jet footprint to explain their spectral signatures and intrinsic $\gamma$-ray luminosities.

This thesis concerns with the study of X-ray binaries which are gravitationally bound systems consisting of a compact object (either a neutron star or a black hole) and usually a non degenerate companion star, both rotating around the common centre of mass. The compact star shines brightly in the X-ray regime. Emission from these systems are powered by accretion which is the most radioactively efficient mechanism known in the universe by the release of gravitational potential energy when matter from the companion star falls on the compact object. Accretion onto high magnetic field neutron stars are special as the magnetic field plays a crucial role in governing the dynamics of gas flow and the flow of the matter close to the compact object. The radiation emitted from these systems are anisotropic and for a distant observer, the intensity is modulated at the spin period of the neutron star, hence these objects are called accretion powered pulsars. The angular pattern of the emitted radiation is also highly anisotropic and depends on the mass accreted and hence the luminosity. The beaming pattern commonly known as the pulse profiles exhibit a wide variety in the pulse shape and pulse fraction and vary with energy as well as intensity. They also exhibit cyclotron absorption features in their energy spectrum which are a direct probe to the magnetic field geometry of these systems. This thesis is dedicated to the study of the magnetic field and emission geometry of accretion powered pulsars through the pulse phase resolved studies of the cyclotron absorption features which are a direct probe of the magnetized plasma. In order to study these features in detail broadband continuum modeling of the energy spectrum is done, taking care of all other factors which may smear the pulse phase dependence. Another prerequisite for detailed continuum modeling is accounting for the low absorption dips in the pulse profiles of many these sources. The dips are presumably formed by phase locked accretion stream causing partial covering absorption when the stream is along our line of sight towards the emission region. Studying the pulse phase dependence of this partial covering absorber also provides us with important clues on the local environment of the neutron star and the structure of the accretion stream. All of these studies are performed with data from the broadband and most sensitive instruments onboard the Japanese satellite Suzuki. Lastly we provide estimates of the polarization expected to be detected from these sources by a Thomson scattering polarimeter being developed to observe the polarization of X-rays in the energy range of 5--30 keV. Along with the X-ray pulsars, we also make an estimate of the likelihood of detection of X-ray polarization from black hole X-ray binaries in different spectral states. This is a particularly interesting topic as it will play a crucial role in providing additional handles on the magnetic field geometry in accretion powered pulsars as well as constrain the fundamental parameters of a black hole like its spin.

This thesis concerns with the study of X-ray binaries which are gravitationally bound systems consisting of a compact object (either a neutron star or a black hole) and usually a non degenerate companion star, both rotating around the common centre of mass. The compact star shines brightly in the X-ray regime. Emission from these systems are powered by accretion which is the most radioactively efficient mechanism known in the universe by the release of gravitational potential energy when matter from the companion star falls on the compact object. Accretion onto high magnetic field neutron stars are special as the magnetic field plays a crucial role in governing the dynamics of gas flow and the flow of the matter close to the compact object. The radiation emitted from these systems are anisotropic and for a distant observer, the intensity is modulated at the spin period of the neutron star, hence these objects are called accretion powered pulsars. The angular pattern of the emitted radiation is also highly anisotropic and depends on the mass accreted and hence the luminosity. The beaming pattern commonly known as the pulse profiles exhibit a wide variety in the pulse shape and pulse fraction and vary with energy as well as intensity. They also exhibit cyclotron absorption features in their energy spectrum which are a direct probe to the magnetic field geometry of these systems. This thesis is dedicated to the study of the magnetic field and emission geometry of accretion powered pulsars through the pulse phase resolved studies of the cyclotron absorption features which are a direct probe of the magnetized plasma. In order to study these features in detail broadband continuum modeling of the energy spectrum is done, taking care of all other factors which may smear the pulse phase dependence. Another prerequisite for detailed continuum modeling is accounting for the low absorption dips in the pulse profiles of many these sources. The dips are presumably formed by phase locked accretion stream causing partial covering absorption when the stream is along our line of sight towards the emission region. Studying the pulse phase dependence of this partial covering absorber also provides us with important clues on the local environment of the neutron star and the structure of the accretion stream. All of these studies are performed with data from the broadband and most sensitive instruments onboard the Japanese satellite Suzuki. Lastly we provide estimates of the polarization expected to be detected from these sources by a Thomson scattering polarimeter being developed to observe the polarization of X-rays in the energy range of 5--30 keV. Along with the X-ray pulsars, we also make an estimate of the likelihood of detection of X-ray polarization from black hole X-ray binaries in different spectral states. This is a particularly interesting topic as it will play a crucial role in providing additional handles on the magnetic field geometry in accretion powered pulsars as well as constrain the fundamental parameters of a black hole like its spin.

The presence of an accretion disk around a neutron star (NS) or a black hole (BH) is mainly responsible for the observed X-rays from the night sky. NS or BH is a part of the binary systems, and matter flows from a companion star to them. That is why they are categorized as X-ray binaries (XRBs). However, accretion flow around an astrophysical object is not only restricted to XRBs. They are ubiquitous in different astrophysical sites e.g., around a white dwarf (WD), supermassive black hole, young stellar object etc. Nevertheless, the hard X-rays present in the observed spectral energy distribution of XRBs cannot be explained by the so-called standard, geometrically thin, optically thick, Keplerian Shakura-Sunyaev disks (SSDs), which is responsible for the softer spectrum. The hard X-rays are responsible for optically thin, geometrically thick, hot advective accretion disks around the central accretor. Till now, the outer region of XRB and active galactic nucleus (AGN) disks is understood to be colder SSD and inner region to be advective accretion disk, together forming a disk-wind system. Although we try to explore different scientific objectives, throughout this thesis work, our study is centered around hot advective accretion flows around WD, NS and BH.