Introduction

Abstract

We explain the terms associated with the title of the thesis. First, we discuss the general view of accretion processes around compact objects, in particular around black holes. Then, we point out the basic properties of accretion around non-rotating black holes. In the case of black hole physics, a full general relativistic approach is recommended, but it makes the time-dependent hydrodynamic equation, which includes radiative transfer, very complex. This problem is circumvented using a pseudo-Newtonian potential. We briefly discuss the governing equations for fluid dynamical study in a pseudo-Newtonian geometry. Subsequently, we discuss the mathematical aspects of shock waves and their presence in accretion processes. Historical studies of the spherical accretion process through various approaches are briefly presented. We start with the Bondi flow for spherical accretion of a normal star. A qualitative discussion on the development of the disc accretion process is also presented. We then discuss the standard Keplerian disc model. This model explains the nature of the multi-coloured soft X-ray spectrum well but fails to explain the high energy radiation coming from stellar mass black holes and distant Quasars and AGNs. This brings advective flows into the picture. This component has lower angular momentum than a Keplerian disc, and is called a sub-Keplerian flow. A realistic accretion flow may have both components, a sub-Keplerian flow surrounding and a Keplerian flow. This is the so-called two-component advective flow or TCAF model of Chakrabarti and Titarchuk.