Effect of matter geometry on low angular momentum black hole accretion in the Kerr metric

TL;DR

This paper investigates how different geometrical configurations of low-angular-momentum accretion flows in the Kerr metric influence shock formation and properties near black holes, using an analytical approach to understand observational phenomena.

Contribution

It introduces an eigenvalue-based analytical method to describe stationary transonic accretion solutions across various geometries without heavy numerical computations.

Findings

Shock formation depends on matter geometry and black hole spin.

Flow behavior near the event horizon varies with geometry and spin.

Shocks in isothermal accretion can power observable flares.

Abstract

This work illustrates how the formation of energy-preserving shocks for polytropic accretion and temperature-preserving shocks for isothermal accretion are influenced by various geometrical configurations of general relativistic, axisymmetric, low-angular-momentum flow in the Kerr metric. Relevant pre- and post-shock states of the accreting fluid, both dynamical and thermodynamic, have been studied comprehensively. Self-gravitational back-reaction on the metric has not been considered in the present context. An elegant eigenvalue-based analytical method has been introduced to provide qualitative descriptions of the phase-orbits corresponding to stationary transonic accretion solutions, without resorting to involved numerical schemes. Effort has been made to understand how the weakly-rotating flow behaves in close proximity of the event horizon and how such `quasi-terminal' quantities are influenced by the black hole spin for different matter geometries. Our main purpose is thus to mathematically demonstrate that for non-self-gravitating accretion, separate matter geometries, in addition to the corresponding space-time geometry, control various shock-induced phenomena observed within black hole accretion discs. This work is expected to reveal how such phenomena observed near the horizon depend on physical environment of the source harbouring a supermassive black hole at its centre. It is also expected to unfold correspondences between the dependence of accretion-related parameters on flow geometries and on black hole spin. Temperature-preserving shocks in isothermal accretion may appear bright as substantial amount of rest-mass energy of the infalling matter gets dissipated at the shock surface, and the prompt removal of such energy to maintain isothermality may power the X-ray/IR flares emitted from our Galactic centre.