Simulations of Viscous Accretion Flow around Black Holes in Two-Dimensional Cylindrical Geometry

TL;DR

This study uses high-accuracy simulations to explore viscous accretion flows around black holes, revealing shock oscillations linked to QPOs and episodic jet emissions, with viscosity influencing shock stability and jet strength.

Contribution

It provides detailed simulation results of viscous accretion flows in cylindrical geometry, demonstrating shock instability and oscillations as sources of QPOs and jets, aligning with theoretical predictions.

Findings

Shock oscillations can produce QPOs in X-ray emissions.

Higher viscosity leads to stronger, faster jets.

Shock instability increases with viscosity, causing transient shocks.

Abstract

We simulate shock-free and shocked viscous accretion flow onto a black hole in a two dimensional cylindrical geometry, where initial conditions were chosen from analytical solutions. The simulation code used the Lagrangian Total Variation Diminishing (LTVD) and remap routine, which enabled us to attain high accuracy in capturing shocks and to handle the angular momentum distribution correctly. Inviscid shock-free accretion disk solution produced a thick disk structure, while the viscous shock-free solution attained a Bondi-like structure, but in either case, no jet activity nor any QPO-like activity developed. The steady state shocked solution in the inviscid, as well as, in the viscous regime, matched theoretical predictions well. However, increasing viscosity renders the accretion shock unstable. Large amplitude shock oscillation is accompanied by intermittent, transient inner multiple shocks. Such oscillation of the inner part of disk is interpreted as the source of QPO in hard X-rays observed in micro-quasars. Strong shock oscillation induces strong episodic jet emission. The jets also showed existence of shocks, which are produced as one shell hits the preceding one. The periodicity of jets and shock oscillation were similar. The jets for higher viscosity parameter are evidently stronger and faster.