Three-dimensional, global, radiative GRMHD simulations of a thermally unstable disc

TL;DR

This study uses 3D general relativistic radiation magnetohydrodynamics simulations to investigate the thermal stability of thin accretion discs around black holes, revealing collapse in radiation pressure dominated discs and stability in gas-pressure dominated ones.

Contribution

First detailed 3D GRRMHD simulations comparing thermal stability of radiation and gas-pressure dominated accretion discs around black holes.

Findings

Radiation pressure dominated disc collapses vertically due to cooling.

Gas-pressure dominated disc remains stable over extended simulation.

Evidence of viscous instability causing ring formation in radiation pressure dominated disc.

Abstract

We present results of a set of three-dimensional, general relativistic radiation magnetohydrodynamics simulations of thin accretion discs around a non-rotating black hole to test their thermal stability. We consider two cases, one that is initially radiation pressure dominated and expected to be thermally unstable and another that is initially gas-pressure dominated and expected to remain stable. Indeed, we find that cooling dominates over heating in the radiation pressure dominated model, causing the disc to collapse vertically on roughly the local cooling timescale. We also find that heating and cooling within the disc have a different dependence on the mid-plane pressure, a prerequisite of thermal instability. Comparison of our data with the relevant thin-disc thermal equilibrium curve suggests that our disc may be headed for the thermally stable, gas-pressure-dominated branch. However, because the disc collapses to the point that we are no longer able to resolve it, we had to terminate the simulation. On the other hand, the gas pressure dominated model, which was run for twice as long as the radiation pressure dominated one, remains stable, with heating and cooling roughly in balance. Finally, the radiation pressure dominated simulation shows some evidence of viscous instability. The strongest evidence is in plots of surface density, which show the disc breaking up into rings.