Is Quasar Optical Variability a Damped Random Walk?
  The damped random walk (DRW) model is increasingly used to model the
variability in quasar optical light curves, but it is still uncertain whether
the DRW model provides an adequate description of quasar optical variability
across all time scales. Using a sample of OGLE quasar light curves, we consider
four modifications to the DRW model by introducing additional parameters into
the covariance function to search for deviations from the DRW model on both
short and long time scales. We find good agreement with the DRW model on time
scales that are well sampled by the data (from a month to a few years),
possibly with some intrinsic scatter in the additional parameters, but this
conclusion depends on the statistical test employed and is sensitive to whether
the estimates of the photometric errors are correct to within ~10%. On very
short time scales (below a few months), we see some evidence of the existence
of a cutoff below which the correlation is stronger than the DRW model, echoing
the recent finding of Mushotzky et al. (2011) using quasar light curves from
Kepler. On very long time scales (> a few years), the light curves do not
constrain models well, but are consistent with the DRW model.
