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

Using a sample of over 25000 spectroscopically confirmed quasars from the Sloan Digital Sky Survey, we show how quasar variability in the rest frame optical/UV regime depends upon rest frame time lag, luminosity, rest wavelength, redshift, the presence of radio and X-ray emission, and the presence of broad absorption line systems. Imaging photometry is compared with threeband spectrophotometry obtained at later epochs spanning time lags up to about two years. The large sample size and wide range of parameter values allow the dependence of variability to be isolated as a function of many independent parameters. The time dependence of variability (the structure function) is well-fit by a single power law with an index γ = 0.246±0.008, on timescales from days to years. There is an anti-correlation of variability amplitude with rest wavelength – e.g. quasars are about twice as variable at 1000˚A as 6000˚A – and quasars are systematically bluer when brighter at all redshifts. There is a strong anti-correlation of variability with quasar luminosity – variability amplitude decreases by a factor of about four when luminosity increases by a factor of 100. There is also a significant positive correlation of variability amplitude with redshift, indicating evolution of the quasar population or the variability mechanism. We parameterize