The Frequency of Extreme X-ray Variability in Bright Quasars

We analyze serendipitous Chandra observations of optically bright quasars to constrain better the frequency of extreme X-ray variability events among typical quasars. Typical quasars are required to be radio-quiet and devoid of broad absorption lines in their optical/UV spectrum. Our quasar sample is mainly drawn from the Sloan Digital Sky Survey data release 14 quasar catalog; however, we include spectroscopically confirmed quasars from the 2QZ and LAMOST surveys that have no SDSS counterpart. We find 472 bright (i ≤ 20.2) quasars with spectroscopic redshifts out to z=5 that have multi-epoch observations in the Chandra data archive. The Chandra observations are separated by up to 6900 days in the rest frame, allowing us to sample changes in X-ray flux over very long timescales. This large sample of quasars allows us to statistically define the X-ray flux ratio that signifies an extreme change with respect to the average quasar. We find that most quasars vary by less than a factor of two in X-ray flux, and that extreme X-ray variability events are very rare among typical quasars. Extreme variability also tends to occur more frequently at larger timescales (dt>1000 days). Finally, we examine the dependence of extreme X-ray variability on different physical properties of these quasars.