A characteristic optical variability timescale in astrophysical accretion disks

TL;DR

This paper identifies a universal optical variability timescale in accretion disks around black holes and white dwarfs, correlating with black hole mass and consistent with thermal timescales predicted by accretion theory.

Contribution

It presents the first measurement of a characteristic variability timescale across a wide range of supermassive black hole masses, linking observations to accretion disk physics.

Findings

The variability timescale correlates with black hole mass.

The timescale matches the thermal timescale at the UV-emitting radius.

White dwarfs follow the same correlation, indicating a common accretion process.

Abstract

Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of timescales. We measure the optical continuum variability observed in 67 active galactic nuclei and the characteristic timescale at which the variability power spectrum flattens. We find a correlation between this timescale and the black hole mass, extending over the entire mass range of supermassive black holes. This timescale is consistent with the expected thermal timescale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks.