Structure Function Analysis of Long-Term Quasar Variability

In our second paper on long-term quasar variability, we employ a much larger database of quasars than in de Vries, Becker & White. This expanded sample, containing 35,165 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 2 and 6413 additional quasars in the same area of the sky taken from the Two Degree Field (2dF) QSO Redshift Survey, allows us to significantly improve on our earlier conclusions. As before, all the historic quasar photometry has been calibrated onto the SDSS scale by using large numbers of calibration stars around each quasar position. We find the following: (1) The outbursts have an asymmetric light-curve profile, with a fast-rise, slow-decline shape; this argues against a scenario in which microlensing events along the line of sight to the quasars are dominating the long-term variations in quasars. (2) There is no turnover in the structure function of the quasars up to timescales of ~40 yr, and the increase in variability with increasing time lags is monotonic and constant. (3) Consequently, there is not a single preferred characteristic outburst timescale for the quasars, but most likely a continuum of outburst timescales. (4) The magnitude of the quasar variability is a function of wavelength: variability increases toward the blue part of the spectrum. (5) High-luminosity quasars vary less than low-luminosity quasars, consistent with a scenario in which variations have limited absolute magnitude. On this basis, we conclude that quasar variability is intrinsic to the active galactic nucleus and is caused by chromatic outbursts or flares that have limited luminosity range, varying timescales, and an overall asymmetric light-curve shape. Currently, the model that has the most promise of fitting the observations is based on accretion disk instabilities.

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