| Abstract : | Black hole binaries are one of the brightest sources in the X-ray sky. Most of the discovered black hole binaries are transient and are detected when they undergo an outburst. Observing these sources in soft and hard X-rays provides a unique view into the processes in the vicinity of the black hole (BH). Some of the fascinating problems in understanding the BH binaries are a) Intrinsic parameters of the BH and their effect on the dynamics of the accretion disk, b) Origin of different variability features, mainly Quasi-Periodic Oscillations (QPOs), and the relation between these temporal features and the emission from these sources.
In this thesis, I study a set of bright BH binaries using X-ray observations conducted by AstroSat, NICER, and NuSTAR to investigate their spectral properties and variability.
I developed a novel method to estimate the spin parameter of the BH using the peak frequencies of the timing features observed in the power density spectrum and applying the Relativistic precession model to the frequencies. I tested the method on the NICER observations of MAXI J1820+070 and estimated the spin.
I studied the spectro-timing properties of a recently discovered BH binary MAXI J1535-571 using AstroSat. In the 5-day-long observation of the source, a strong correlation between the spectral slope and the QPO frequency and a lack of correlation between QPO frequency and the flux was observed, indicating a strong association of the QPO with the comptonising medium.
I developed a code to identify the unique temporal features (shots) observed in BH binary Cyg X-1. I investigated the spectral properties of these features in a joint NICER-AstroSat observation to understand their origin.
In my studies, I have identified unique features in the spectro-timing properties of BH binaries that challenge the standard picture of the accretion around the BH. |
