Time Evolution of the 3-D Accretion Flows: Effects of the Adiabatic Index and Outer Boundary Condition

TL;DR

This study uses 3-D simulations to explore how the adiabatic index and boundary conditions influence accretion flows onto black holes, revealing consistent precession of the inner torus across different gamma values and boundary scenarios.

Contribution

It extends previous 3-D accretion flow models by analyzing the effects of various adiabatic indices and boundary conditions on torus precession and flow behavior.

Findings

Inner torus precesses for gamma=5/3, 4/3, 1.2, 1.01

Precession time increases as gamma decreases

Boundary conditions can suppress or enable torus precession

Abstract

We study a slightly rotating accretion flow onto a black hole, using the fully three dimensional (3-D)numerical simulations. We consider hydrodynamics of an inviscid flow, assuming a spherically symmetric density distribution at the outer boundary and a small, latitude-dependent angular momentum. We investigate the role of the adiabatic index and gas temperature, and the flow behaviour due to non-axisymmetric effects. Our 3-D simulations confirm axisymmetric results: the material that has too much angular momentum to be accreted forms a thick torus near the equator and the mass accretion rate is lower than the Bondi rate. In our previous study of the 3-D accretion flows, for gamma=5/3, we found that the inner torus precessed, even for axisymmetric conditions at large radii. The present study shows that the inner torus precesses also for other values of the adiabatic index: gamma=4/3, 1.2 and 1.01. However, the time for the precession to set increases with decreasing gamma. In particular, for gamma=1.01 we find that depending on the outer boundary conditions, the torus may shrink substantially due to the strong inflow of the non-rotating matter and the precession will have insufficient time to develop. On the other hand, if the torus is supplied by the continuous inflow of the rotating material from the outer radii, its inner parts will eventually tilt and precess, as it was for the larger gamma's.