Spectral and Timing Studies of Accretion Disk in Black Hole Binaries

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dc.contributor.advisor Vadawale, Santosh V.
dc.contributor.author Jassal, Anjali Rao
dc.date.accessioned 2016-02-20T09:01:58Z
dc.date.available 2016-02-20T09:01:58Z
dc.date.issued 2015
dc.identifier.citation Jassal, Anjali Rao (2015). Spectral and Timing Studies of Accretion Disk in Black Hole Binaries (PhD Thesis). Indian Institute of Technology, Gandhinagar, pp. 141 (Acc No: T00099) en_US
dc.identifier.uri https://repository.iitgn.ac.in/handle/123456789/2106
dc.description.abstract Accretion around compact objects is the most efficient way of extracting energy from material in an accretion disk. Luminous accretion disks around black holes in mass transferring black hole binaries offer an opportunity to study accretion processes taking place in the extreme conditions of temperature and gravitational field. However, these astrophysical sources cannot be observed with ground based conventional telescopes because they emit mainly in X-rays. Therefore, data are collected with space borne X-ray observatories. Spectra, light curves and images extracted from data are used as probes to understand the radiative processes in accretion disks. Although a significant understanding of the sources has been developed with the help of several empirical and theoretical studies, there are several phenomena, which are not understood. This thesis presents the efforts that have been made to understand some of these phenomena. A majority of black hole binaries are transient in nature and they keep on switching between quiescent and outburst states. It is generally believed that an accretion disk is truncated far away from the central black hole during quiescent states. However, the observational evidence for this general picture is indirect at best. We studied a transient black hole candidate MAXI J1659−152 during its 2010 outburst, which was detected during very early stages of the outburst. We investigated the variation of the inner disk radius with progress of the outburst and found that the disk is truncated at larger radii in the beginning. A systematic decrease in the inner disk radius was found as the source transitions towards the soft states. We estimated mass of the black hole to be 8.1±2.9 M⊙ with the help of normalization of the disk blackbody component. A transient black hole candidate IGR J17091−3624 is studied, which behaved like ‘normal’ black hole binaries except during its latest outburst in 2011. The source showed properties similar to a unique black hole binary GRS 1915+105, known for exhibiting extreme variability. High mass accretion rate is believed to give rise to extreme variability, therefore IGR J17091−3624 is expected to be a bright source. However, IGR J17091−3624 is about 20 times fainter as compared to GRS 1915+105. We performed a comparative study of the two sources by investigating ‘heartbeat’-type variability observed in their light curves. The light curves were folded and spectra were generated for the 5 phases of ‘heartbeat’ oscillations. The spectra were fitted simultaneously by tying the system parameters and leaving the accretion-process-dependent parameters independently free across the 5 phases. We found important constraints on mass, distance, inclination and spin of IGR J17091−3624. It was noticed that the estimated value of spin is low (a∗ < 0.2) as opposed to the high value of spin for GRS 1915+105. We suggest that the low spin of the black hole in IGR J17091−3624 can be a reason behind its faintness as compared to GRS 1915+105 instead of showing variability patterns arising from high mass accretion rate. Black hole binaries are known for showing quasi periodic oscillations (QPOs) during hard states, which appear as narrow peaks superimposed on the broad band continuum in power density spectrum. Several studies have shown the correlation between QPO properties and spectral parameters, indicating a close link between the two. However, the mechanism behind generation of QPOs is not well understood. The thesis presents our attempts to understand the QPO mechanism by simulating light curves. We studied a NuSTAR observation of GRS 1915+105, which has a power law dominated spectrum and shows the presence of reflection component. Since there is only one primary component, we make a hypothesis that the QPO is generated as a result of oscillation of some of the spectral parameters (instead of spectral components) at frequencies close to that of the QPO. We test the hypothesis by finding whether the simulated results explain the observed energy dependence of QPO or not. It was found that the observed trend of increasing QPO power with energy can be reproduced qualitatively if the spectral index is varied with the phases of QPO. Variation of other spectral parameters does not reproduce the observed energy dependence. The variation of spectral index is verified by performing phase-resolved spectroscopy for the phases of QPO. The results clearly show the variation of spectral index with the phases of QPO. The finding of variation of spectral index is an important result and it puts significant constraints on the models explaining modulation mechanism for QPOs. en_US
dc.description.statementofresponsibility by Anjali Rao Jassal
dc.format.extent 141 p.; col.; ill; 24 cm. + 1 CD-ROM
dc.language.iso en_US en_US
dc.publisher Indian Institute of Technology Gandhinagar en_US
dc.subject Accretion en_US
dc.subject accretion disks en_US
dc.subject black hole physics en_US
dc.subject X-rays: binaries, X-rays: individual (GRS 1915+105, MAXI J1659−152, IGR J17091−3624) en_US
dc.title Spectral and Timing Studies of Accretion Disk in Black Hole Binaries en_US
dc.type Thesis en_US
dc.contributor.department Physics
dc.description.degree Ph.D.

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