  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.
