Studies of dissipative standing shock waves around black holes

TL;DR

This study explores the structure and behavior of shock waves in accretion flows around black holes, highlighting how energy dissipation influences shock stability and the generation of high-frequency QPOs observed in black hole emissions.

Contribution

It identifies the parameter space for dissipative shocks in accretion flows and demonstrates how increased dissipation affects shock stability and oscillations, linking to observed QPO phenomena.

Findings

Dissipative shocks exist within a specific parameter space.

Increased dissipation reduces the shock parameter space.

Oscillating shocks can produce high-frequency QPOs.

Abstract

We investigate the dynamical structure of advective accretion flow around stationary as well as rotating black holes. For a suitable choice of input parameters, such as, accretion rate ($\dot {\cal M}$) and angular momentum ($\lambda$), global accretion solution may include a shock wave. The post shock flow is located at few tens of Schwarzchild radius and it is generally very hot and dense. This successfully mimics the so called Compton cloud which is believed to be responsible for emitting hard radiations. Due to the radiative loss, a significant energy from the accreting matter is removed and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of the parameter space ($\dot {\cal M} - \lambda$) which allows accretion flows to have some energy dissipation at the shock $(\Delta {\cal E})$. As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation is reached its critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock whose oscillation causes Quasi-Periodic Oscillations (QPOs) in the emitted radiation, will produce oscillations of high frequency. Such an evolution of QPOs has been observed in several black hole candidates during their outbursts.