A characteristic optical variability time scale in astrophysical accretion disks

Abstract

Variability time scales in active galaxies Active galactic nuclei contain a supermassive black hole (SMBH) surrounded by an accretion disk. As disk material falls toward the SMBH, it heats up enough to emit optical light. Burke et al. investigated how such optical emission varies over time in a sample of 67 active galaxies (see the Perspective by Lira and Arevalo). They observed a characteristic variability in timing that scaled with the SMBH mass. The results elucidate the physical processes within accretion disks and provide a method to estimate SMBH mass from optical variability observations. Science, abg9933, this issue p. 789; see also abk3451, p. 734 Optical variability of active galactic nuclei provides a method to estimate supermassive black hole masses. 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 time scales. We measured the optical continuum variability observed in 67 active galactic nuclei and the characteristic time scale at which the variability power spectrum flattens. We found a correlation between this time scale and the black hole mass extending over the entire mass range of supermassive black holes. This time scale is consistent with the expected thermal time scale 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.