Long-term variability of Sloan Digital Sky Survey quasars

We use a sample of 3791 quasars from the Sloan Digital Sky Survey (SDSS) Early Data Release and compare their photometry with historic plate material for the same set of quasars in order to study their variability properties. The time baseline we attain this way ranges from a few months to up to 50 yr. In contrast to monitoring programs, where relatively few quasars are photometrically measured over shorter time periods, we use existing databases to extend this baseline as much as possible, at the cost of sampling per quasar. Our method, however, can easily be extended to much larger samples. We construct variability structure functions and compare these with the literature and model functions. From our modeling, we conclude that ( 1) quasars are more variable toward shorter wavelengths, ( 2) their variability is consistent with an exponentially decaying light curve with a typical timescale of similar to 2 yr, and ( 3) these outbursts occur on typical timescales of similar to 200 yr. With the upcoming first data release of the SDSS, a much larger quasar sample can be used to put these conclusions on a more secure footing.