Hydrodynamic simulations of viscous accretion flows around black holes

TL;DR

This paper investigates how viscosity affects shock formation, flow structure, and outflows in viscous accretion flows around black holes through hydrodynamic simulations, revealing critical viscosity thresholds that alter flow regimes and spectral states.

Contribution

It provides a detailed analysis of the impact of viscosity on shock dynamics, flow topology, and outflow rates in black hole accretion disks, including comparisons of different viscous stress components.

Findings

Shocks weaken and form farther out with increased viscosity.

Above a critical viscosity, shocks disappear, leading to subsonic, Keplerian flows.

Outflows decrease significantly as viscosity increases.

Abstract

We study the time evolution of a rotating, axisymmetric, viscous accretion flow around black holes using a grid based finite difference method. We use the Shakura-Sunyaev viscosity prescription. However, we compare with the results obtained when all the three independent components of the viscous stress are kept. We show that the centrifugal pressure supported shocks became weaker with the inclusion of viscosity. The shock is formed farther out when the viscosity is increased. When the viscosity is above a critical value, the shock disappears altogether and the flow becomes subsonic and Keplerian everywhere except in a region close to the horizon, where it remains supersonic. We also find that as the viscosity is increased, the amount of outflowing matter in the wind is decreased to less than a percentage of the inflow matter. Since the post-shock region could act as a reservoir of hot electrons or the so-called 'Compton cloud', the size of which changes with viscosity, the spectral properties are expected to depend on viscosity strongly: the harder states are dominated by low-angular momentum and the low viscosity flow with a significant outflows while the softer states are dominated by the high viscosity Keplerian flow having very little outflows.