Opacity Driven Convection and Variability in Accretion Disks around Supermassive Black Holes

TL;DR

This paper investigates how iron opacity bumps induce convection-driven turbulence in accretion disks around supermassive black holes, leading to significant luminosity variability over years to decades.

Contribution

It demonstrates that iron opacity-driven convection causes oscillations in disk structure and luminosity, providing a physical mechanism for AGN variability.

Findings

Convection caused by iron opacity bump leads to turbulence and disk puffing.

Disk luminosity varies by a factor of 3-6 over years.

Opacity changes induce cyclic cooling and heating of the disk.

Abstract

We study the structure of accretion disks around supermassive black holes in the radial range $30\sim 100$ gravitational radii, using a three dimensional radiation magneto-hydrodynamic simulation. For typical conditions in this region of Active Galactic Nuclei (AGN), the Rosseland mean opacity is expected to be larger than the electron scattering value. We show that the iron opacity bump causes the disk to be convective unstable. Turbulence generated by convection puffs up the disk due to additional turbulent pressure support and enhances the local angular momentum transport. This also results in strong fluctuations in surface density and heating of the disk. The opacity drops with increasing temperature and convection is suppressed. The disk cools down and the whole process repeats again. This causes strong oscillations of the disk scale height and luminosity variations by more than a factor of $\approx 3-6$ over a few years' timescale. Since the iron opacity bump will move to different locations of the disk for black holes with different masses and accretion rates, we suggest that this is a physical mechanism that can explain the variability of AGN with a wide range of amplitudes over a time scale of years to decades.