Spectral variability of quasars from multi-epoch photometric data in the
  Sloan Digital Sky Survey Stripe 82
  We present a new approach to analysing the dependence of quasar variability
on rest-frame wavelengths. We exploited the spectral archive of the Sloan
Digital Sky Survey (SDSS) to create a sample of more than 9000 quasars in the
Stripe 82. The quasar catalogue was matched with the Light Motion Curve
Catalogue for SDSS Stripe 82 and individual first-order structure functions
were computed. The structure functions are used to create a variability
indicator that is related to the same intrinsic timescales for all quasars (1
to 2 yr in the rest frame). We study the variability ratios for adjacent SDSS
filter bands as a function of redshift. While variability is almost always
stronger in the bluer passband compared to the redder, the variability ratio
depends on whether strong emission lines contribute to either one band or the
other. The variability ratio-redshift relations resemble the corresponding
colour index-redshift relations. From the comparison with Monte Carlo
simulations of variable quasar spectra we find that the observed variability
ratio-redshift relations are closely fitted assuming that (a) the r.m.s.
fluctuation of the quasar continuum follows a power law-dependence on the
intrinsic wavelength with an exponent -2 (i.e., bluer when brighter) and (b)
the variability of the emission line flux is only about 10% of that of the
underlying continuum. These results, based upon the photometry of more than
8000 quasars, confirm the previous findings by Wilhite et al. (2005) from 315
quasars with repeated SDSS spectroscopy. Finally, we find that quasars with
unusual spectra and weak emission lines tend to have less variability than
conventional quasars. This trend is opposite to what is expected from the
dilution effect of variability due to line emission and may be indicative of
high Eddington ratios in these unconventinal quasars.
