Sree Suribhatla, BS (Honours), Class of 2023

Supervisor: Professor Pankaj Joshi, School of Arts and Sciences

Thesis Brief

Accretion processes have garnered a lot of attention in recent years, from playing a key role in star formation to forming discs around compact objects. Theories related to accretion processes, initially formulated by Novikov & Thorne (1973) [18] and Shakura & Sunyaev (1973) [23], provided much-required knowledge to understand quasars, X-ray binaries, and active galactic nuclei. One of the primary assumptions of these theories is that internal energy generated due to viscous forces is emitted in the form of radiation. This radiation’s frequency range is dependent on the mass of the central body onto which accretion is happening. Accretion disks of young stars and protostars emit radiation in the infrared spectrum while those of neutron stars and black holes emit mostly in the X-ray spectrum. Further, the concept of Quasiperiodic oscillations (QPOs) tries to understand how these X-rays are emitted from the accretion disc. When gas is orbiting a compact object X-rays are usually emitted from the inner regions of the accretion disk. This paper aims to study accretion discs in X-ray binary sources with black holes as the heavier, central bodies, commonly known as Blackhole X-ray Binaries (BHXRBs). It begins with understanding fundamental concepts related to accretion discs and radiative processes in accretion disks within the framework of astrophysics. These concepts include radiative processes and gas dynamics, accretion disk formation in BHXRBs, QPOs, their classifications, and other related aspects. This paper also involves timing and spectral analysis of the low-mass X-ray binary (LMXB) with a black hole candidate GX 339-4 using the observations from AstroSat - India’s multiwavelength observatory.