Quasar Optical Variability in the Palomar-QUEST Survey
  The ensemble variability properties of nearly 23,000 quasars are studied
using the Palomar-QUEST Survey. The survey has covered 15,000 square degrees
multiple times over 3.5 years using 7 optical filters, and has been calibrated
specifically for variability work. Palomar-QUEST allows for the study of rare
objects using multiple epochs of consistently calibrated, homogeneous data,
obviating the common problem of generating comparable measurements from
disparate datasets. A power law fit to the quasar structure function versus
time yields an index of 0.432 +/- 0.024 for our best measured sample. We see
the commonly reported anticorrelation between average optical variability
amplitude and optical luminosity, and measure the logarithmic decrease in
variability amplitude to scale as the logarithm of the luminosity times 0.205
+/- 0.002. Black hole mass is positively correlated with variability amplitude
over three orders of magnitude in mass. Quasar variability amplitude is seen to
decrease with Eddington ratio as a step function with a transition around
Eddington ratio of 0.5. The higher variability at low Eddington ratios is due
to excess power at timescales shorter than roughly 300 days. X-ray and radio
measurements exist for subsets of the quasar sample. We observe an
anticorrelation between optical variability amplitude and X-ray luminosity. No
significant correlation is seen between average optical variability properties
and radio luminosity. The timescales of quasar fluctuations are suggestive of
accretion disk instabilities. The relationships seen between variability,
Eddington ratio, and radio and X-ray emission are discussed in terms of a
possible link between the behavior of quasars and black hole X-ray binaries.
