Radiation-Dominated Disks Are Thermally Stable

TL;DR

This study uses 3D radiation MHD simulations to demonstrate that radiation-dominated accretion disks can be thermally stable, challenging previous expectations of instability in such regions.

Contribution

The paper provides the first simulation evidence that radiation-dominated disks can remain thermally stable, with magnetic fluctuations driving pressure variations.

Findings

No thermal runaway observed over 40 cooling times.

Radiation flux remains at critical value despite high radiation pressure.

Magnetic fluctuations lead pressure variations, stabilizing the disk.

Abstract

When the accretion rate is more than a small fraction of Eddington, the inner regions of accretion disks around black holes are expected to be radiation-dominated. However, in the alpha-model, these regions are also expected to be thermally unstable. In this paper, we report two 3-d radiation MHD simulations of a vertically-stratified shearing box in which the ratio of radiation to gas pressure is ~ 10, and yet no thermal runaway occurs over a timespan ~ 40 cooling times. Where the time-averaged dissipation rate is greater than the critical dissipation rate that creates hydrostatic equilibrium by diffusive radiation flux, the time-averaged radiation flux is held to the critical value, with the excess dissipated energy transported by radiative advection. Although the stress and total pressure are well-correlated as predicted by the alpha-model, we show that stress fluctuations precede pressure fluctuations, contrary to the usual supposition that the pressure controls the saturation level of the magnetic energy. This fact explains the thermal stability. Using a simple toy-model, we show that independently-generated magnetic fluctuations can drive radiation pressure fluctuations, creating a correlation between the two while maintaining thermal stability.