Segregation of a Keplerian disc and sub-Keplerian halo from a Transonic flow around a Black Hole by Viscosity and Cooling processes

TL;DR

This study uses numerical simulations to demonstrate how viscosity and cooling effects cause a black hole accretion flow to segregate into a Keplerian disc and a sub-Keplerian halo, revealing complex flow structures near the event horizon.

Contribution

It provides the first detailed simulation showing the formation of a Keplerian disc and halo segregation due to variable viscosity and cooling in transonic flows around black holes.

Findings

Flow segregates into Keplerian and sub-Keplerian components.

Keplerian disc extends to the horizon without truncation.

Presence of oscillating shock waves around the disc.

Abstract

A black hole accretion is necessarily transonic. In presence of sufficiently high viscosity and cooling effects, a low-angular momentum transonic flow can become a standard Keplerian disc except close to the where hole where it must pass through the inner sonic point. However, if the viscosity is not high everywhere and cooling is not efficient everywhere, the flow cannot completely become a Keplerian disc. In this paper, we show results of rigorous numerical simulations of a transonic flow having vertically varying viscosity parameter (being highest on the equatorial plane) and optical depth dependent cooling processes to show that the flow indeed segregates into two distinct components as it approaches a black hole. The component on the equatorial plane has properties of a standard Keplerian disc, though the flow is not truncated at the inner- most stable circular orbit. This component extends till the horizon as a sub-Keplerian flow. This standard disc is found to be surrounded by a hot, low angular momentum component forming a centrifugal barrier dominated oscillating shock wave, consistent with the Chakrabarti-Titarchuk two component advective flow configuration.