Accretion of a Vlasov gas on to a black hole from a sphere of finite radius and the role of angular momentum

TL;DR

This paper models the accretion of collisionless gas onto a black hole from a finite radius sphere, incorporating angular momentum and relativistic effects, to better understand low-luminosity supermassive black holes like Sgr A* and M87*.

Contribution

It introduces a novel approach by analyzing gas accretion from a finite radius boundary with angular momentum, extending previous models that used boundary conditions at infinity.

Findings

Mass accretion rates are computed for various models and compared.

Results align with observational bounds for Sgr A* and M87*.

Angular momentum influences accretion and luminosity estimates.

Abstract

The accretion of a spherically symmetric, collisionless kinetic gas cloud on to a Schwarzschild black hole is analysed. Whereas previous studies have treated this problem by specifying boundary conditions at infinity, here the properties of the gas are given at a sphere of finite radius. The corresponding steady-state solutions are computed using four different models with an increasing level of sophistication, starting with the purely radial infall of Newtonian particles and culminating with a fully general relativistic calculation in which individual particles have angular momentum. The resulting mass accretion rates are analysed and compared with previous models, including the standard Bondi model for a hydrodynamic flow. We apply our models to the supermassive black holes Sgr A* and M87*, and we discuss how their low luminosity could be partially explained by a kinetic description involving angular momentum. Furthermore, we get results consistent with previous model-dependent bounds for the accretion rate imposed by rotation measures of the polarised light coming from Sgr A* and with estimations of the accretion rate of M87* from the Event Horizon Telescope collaboration. Our methods and results could serve as a first approximation for more realistic black hole accretion models in various astrophysical scenarios in which the accreted material is expected to be nearly collisionless.