Is Quasar Optical Variability a Damped Random Walk?

TL;DR

This study tests the damped random walk model for quasar optical variability across different time scales, finding it generally adequate but with some deviations at very short time scales, based on OGLE data.

Contribution

The paper evaluates the DRW model against modified models using OGLE quasar light curves, highlighting its adequacy and limitations across various time scales.

Findings

Good agreement with DRW on months to years time scales

Evidence of a cutoff in correlation at short time scales

Long-term data are consistent with DRW model

Abstract

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.