The Ensemble Photometric Variability of ∼25,000 Quasars in the Sloan Digital Sky Survey

Using a sample of over 25000 spectroscopically confirmed quasars from theSloan Digital Sky Survey, we show how quasar variability in the rest frameoptical/UV regime depends upon rest frame time lag, luminosity, restwavelength, redshift, the presence of radio and X-ray emission, and thepresence of broad absorption line systems. The time dependence of variability(the structure function) is well-fit by a single power law on timescales fromdays to years. There is an anti-correlation of variability amplitude with restwavelength, and quasars are systematically bluer when brighter at allredshifts. There is a strong anti-correlation of variability with quasarluminosity. There is also a significant positive correlation of variabilityamplitude with redshift, indicating evolution of the quasar population or thevariability mechanism. We parameterize all of these relationships. Quasars withRASS X-ray detections are significantly more variable (at optical/UVwavelengths) than those without, and radio loud quasars are marginally morevariable than their radio weak counterparts. We find no significant differencein the variability of quasars with and without broad absorption line troughs.Models involving multiple discrete events or gravitational microlensing areunlikely by themselves to account for the data. So-called accretion diskinstability models are promising, but more quantitative predictions are needed.Comment: 41 pages, 21 figures, AASTeX, Accepted for publication in Ap