Spectral Variability of Quasars in the Sloan Digital Sky Survey. I. Wavelength Dependence

Sloan Digital Sky Survey (SDSS) repeat spectroscopic observations haveresulted in multiple-epoch spectroscopy for roughly 2500 quasars observed morethan 50 days apart. From this sample, we identify 315 quasars that have variedsignificantly between observations. We create an ensemble difference spectrum(bright phase minus faint phase) covering rest-frame wavelengths from 1000 to6000 Angstroms. This average difference spectrum is bluer than the averagesingle-epoch quasar spectrum; a power-law fit to the difference spectrum yieldsa spectral index alpha_lambda = -2.00, compared to an index of alpha_lambda =-1.35 for the single-epoch spectrum. The strongest emission lines vary only 30%as much as the continuum. Due to the lack of variability of the lines, measuredphotometric color is not always bluer in brighter phases, but depends onredshift and the filters used. Lastly, the difference spectrum is bluer thanthe ensemble quasar spectrum only for lambda_rest < 2500 Angstroms, indicatingthat the variability cannot result from a simple scaling of the average quasarspectrum.Comment: 47 pages, 14 figures, 3 tables, accepted for publication in Ap