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

• Published in: The Astrophysical journal 2004-02, Vol.601 (2), p.692-714
• Main Authors: Vanden Berk, Daniel E, Wilhite, Brian C, Kron, Richard G, Anderson, Scott F, Brunner, Robert J, Hall, Patrick B, Ivezić, Željko, Richards, Gordon T, Schneider, Donald P, York, Donald G, Brinkmann, Jonathan V, Lamb, Don Q, Nichol, Robert C, Schlegel, David J
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/380563

Using a sample of over 25,000 spectroscopically confirmed quasars from the Sloan Digital Sky Survey, we show how quasar variability in the rest-frame optical/UV regime depends on 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 three-band spectrophotometry obtained at later epochs spanning time lags up to about 2 yr. 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 fitted by a single power law with an index gamma = 0.246 plus or minus 0.008, on timescales from days to years. There is an anticorrelation of variability amplitude with rest wavelength-e.g. quasars are about twice as variable at 1000 Aa as at 6000 Aa-and quasars are systematically bluer when brighter at all redshifts. There is a strong anticorrelation of variability with quasar luminosity-variability amplitude decreases by a factor of about 4 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 all of these relationships. Quasars with ROSAT All-Sky Survey X-ray detections are significantly more variable (at optical/UV wavelengths) than those without, and radio-loud quasars are marginally more variable than their radio-quiet counterparts. We find no significant difference in the variability of quasars with and without broad absorption line troughs. Currently, no models of quasar variability address more than a few of these relationships. Models involving multiple discrete events or gravitational microlensing are unlikely by themselves to account for the data. So-called accretion disk instability models are promising, but more quantitative predictions are needed.