Time Variability of Quasars: the Structure Function Variance

TL;DR

This study tests the assumption that two-epoch quasar variability data accurately reflect the statistical properties of well-sampled light curves, finding that while two-epoch data capture key trends, detailed variability information requires well-sampled light curves.

Contribution

It provides a critical evaluation of the validity of two-epoch variability studies by comparing them with detailed light curves, highlighting the additional information in well-sampled data.

Findings

Two-epoch and well-sampled light curves show similar mean variability trends.

Variability differs between radio/X-ray selected subsamples.

Scatter in variability is mainly due to intrinsic property variance.

Abstract

Significant progress in the description of quasar variability has been recently made by employing SDSS and POSS data. Common to most studies is a fundamental assumption that photometric observations at two epochs for a large number of quasars will reveal the same statistical properties as well-sampled light curves for individual objects. We critically test this assumption using light curves for a sample of $\sim$2,600 spectroscopically confirmed quasars observed about 50 times on average over 8 years by the SDSS stripe 82 survey. We find that the dependence of the mean structure function computed for individual quasars on luminosity, rest-frame wavelength and time is qualitatively and quantitatively similar to the behavior of the structure function derived from two-epoch observations of a much larger sample. We also reproduce the result that the variability properties of radio and X-ray selected subsamples are different. However, the scatter of the variability structure function for fixed values of luminosity, rest-frame wavelength and time is similar to the scatter induced by the variance of these quantities in the analyzed sample. Hence, our results suggest that, although the statistical properties of quasar variability inferred using two-epoch data capture some underlying physics, there is significant additional information that can be extracted from well-sampled light curves for individual objects.