Model-Dependent Galactic Environment Effects on Multi-Transonic Accretion and Emergent Acoustic Gravity around Kerr Black Holes

Abstract

The hydrodynamics of low angular momentum, multi-transonic, axisymmetric, inviscid accretion flow onto a rotating black hole in presence of a galactic environment has been systematically investigated in detail using three standard disc geometries and two thermodynamic equations of state, within the post-Newtonian framework with a pseudo-potential. In addition to the influence of a centrally located black hole, this study incorporates those due to a multi-component galactic environment -- stellar matter, dark matter, and hot gas. Our analysis reveals that the extent of additional influence of the galactic environment on accretion flow depends severely on the disc model chosen. This effect is particularly pronounced in the vertical equilibrium disc model, whereas it is extremely mild for other two geometries. The same is observed for location of sonic points, parameters related to multi-transonicity and shock formation, and also for the emergent analogue surface gravity calculated. Another observation is that, among different components of galactic environment, contribution of dark matter is most dominant, followed by a mild one due to hot gas; while that due to stellar matter is even milder in determining the flow. The stationary flow characteristics are analysed using semi-analytical numerical methods including the method analogous to critical point analysis used in dynamical systems. Additionally, a time-dependent linear perturbation analysis ensures the stability of stationary accretion flow in all cases, and hence the corresponding acoustic metric and acoustic surface gravity are derived.