The Dynamics of Truncated Black Hole Accretion Disks II: Magnetohydrodynamic Case

TL;DR

This study uses magnetohydrodynamic simulations to explore the flow dynamics of truncated black hole accretion disks, revealing how magnetic activity and accretion efficiency vary across different disk regions.

Contribution

It provides a detailed analysis of the flow dynamics, magnetic properties, and angular momentum transport in truncated accretion disks with a bistable cooling function.

Findings

Magnetic dynamo is suppressed in the hot, inner disk region.

Effective alpha parameter decreases by 65% in the hot, truncated zone.

A transition zone exists between hot inner gas and cold thin disk.

Abstract

We study a truncated accretion disk using a well-resolved, semi-global magnetohydrodynamic simulation that is evolved for many dynamical times (6096 inner disk orbits). The spectral properties of hard state black hole binary systems and low-luminosity active galactic nuclei are regularly attributed to truncated accretion disks, but a detailed understanding of the flow dynamics is lacking. In these systems the truncation is expected to arise through thermal instability driven by sharp changes in the radiative efficiency. We emulate this behavior using a simple bistable cooling function with efficient and inefficient branches. The accretion flow takes on an arrangement where a "transition zone" exists in between hot gas in the inner most regions and a cold, Shakura $\&$ Sunyaev thin disk at larger radii. The thin disk is embedded in an atmosphere of hot gas that is fed by a gentle outflow originating from the transition zone. Despite the presence of hot gas in the inner disk, accretion is efficient. Our analysis focuses on the details of the angular momentum transport, energetics, and magnetic field properties. We find that the magnetic dynamo is suppressed in the hot, truncated inner region of the disk which lowers the effective $\alpha$-parameter by $65\%$.