Influence of flow thickness on general relativistic low angular momentum accretion around spinning black holes

Abstract

General relativistic, axisymmetric flow of low angular momentum accretion around a Kerr black hole can have certain geometric configuration where the flow is maintained in hydrostatic equilibrium along the vertical direction (direction orthogonal to the equatorial plane of the flow). The flow thickness for such accretion models becomes a function of the local radial distance measured from the black hole horizon. There are three types of functions defined in the literature which resemble the thickness of the flow for such a configuration. We formulate the equations governing the steady state astrophysical accretion characterized by both the polytropic as well as the isothermal equation of state in classical thermodynamics. We solve the equations within the framework of such geometric configuration for three different thickness functions, to obtain the multi-transonic, shocked, stationary integral accretion solutions. Such solutions enable us to study how flow thickness influences the dependence of the properties of post-shock flows on black hole spin angular momentum, i.e., the Kerr parameter. For temperature-preserving standings shocks, we find that the post-shock part of the disc can become luminous and considerable amount of gravitational energy carried by the accreting fluid can get liberated at the shock. We find which kind of thickness function produces the maximum liberated energy, making the disc most luminous.