Relativistic Low Angular Momentum Accretion: Long Time Evolution of Hydrodynamical Inviscid Flows

TL;DR

This paper presents relativistic hydrodynamical simulations of low angular momentum accretion onto a Schwarzschild black hole, revealing turbulent behavior, lower accretion rates than radial flows, and the formation of thick tori at low sound speeds.

Contribution

It introduces a high-resolution, shock-capturing numerical code for relativistic accretion, demonstrating qualitative agreement with Newtonian models and exploring the formation of tori and accretion dynamics.

Findings

Turbulent accretion flows observed.

Accretion rates are lower than in radial flow models.

Thick tori form at low sound speeds.

Abstract

We investigate relativistic low angular momentum accretion of inviscid perfect fluid onto a Schwarzschild black hole. The simulations are performed with a general-relativistic, high-resolution (second-order), shock-capturing, hydrodynamical numerical code. We use horizon-penetrating Eddington-Finkelstein coordinates to remove inaccuracies in regions of strong gravity near the black hole horizon and show the expected convergence of the code with the Michel solution and stationary Fishbone-Moncrief toroids. We recover, in the framework of relativistic hydrodynamics, the qualitative behavior known from previous Newtonian studies that used a Bondi background flow in a pseudo-relativistic gravitational potential with a latitude-dependent angular momentum at the outer boundary. Our models exhibit characteristic "turbulent" behavior and the attained accretion rates are lower than those of the Bondi-Michel radial flow. For sufficiently low values of the asymptotic sound speed, geometrically thick tori form in the equatorial plane surrounding the black hole horizon while accretion takes place mainly through the poles.