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

By D. V. vanden Berk, B. Wilhite, R. Kron, S. Anderson, Robert J. Brunner, P. Hall, Željko Ivezic´, Gordon Richards, D. Schneider, D. York and 4 more. 2003

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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 γ = 0.246 ± 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 Å as at 6000 Å—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.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

The Time Delays of Gravitational Lens HE 0435–1223: An Early-Type Galaxy with a Rising Rotation Curve (2005, methodology): “Vanden Berk et al. (2004) have measured the typical quasar structure function in the r band, using a large ensemble of quasar photometry from the SDSS, finding V ð Þ ¼ = 0ð Þ mag2; ð4Þ with 0 ’ 70; 000 days and ’ 1/3.”
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