NED Abstract 
Copyright by European Southern Observatory (ESO).
Reproduced by permission

2011A&A...525A..37M

Spectral variability of quasars from multi-epoch photometric data in the
Sloan Digital Sky Survey Stripe 82

Meusinger, H.; Hinze, A.; de Hoon, A.

Abstract.  Aims: The study of the ensemble properties of the UV/optical
broadband variability of quasars is hampered by the combined effects of
the dependence of variability on timescale, rest-frame wavelength, and
luminosity. Here, we present a new approach to analysing the dependence of
quasar variability on rest-frame wavelengths.  Methods: We exploited the
spectral archive of the Sloan Digital Sky Survey (SDSS) to create a sample
of over 9000 quasars in the Stripe 82. The quasar catalogue was matched
with the Light Motion Curve Catalogue for SDSS Stripe 82 and first-order
structure functions were computed from the lightcurves. The structure
functions are used to create a variability indicator that is related to
the same intrinsic timescales for all quasars (about 1 to 2 yr in the
rest-frame). We study the variability ratios for adjacent SDSS filter
bands as a function of redshift. A quantitative interpretation of these
relations is provided by comparing with the results of simple Monte Carlo
simulations of variable quasar spectra.  Results: We confirm the
well-known dependence of variability on time-lag; the best power-law fit
of the sample-averaged structure function has a slope {beta} = 0.31 +/-
0.03. We also confirm that anti-correlations exist with luminosity,
wavelength, and redshift, where the latter can be fully explained as a
consequence of the former two dependencies. The variability ratios as a
function of redshift resemble the corresponding colour index-redshift
relations. While variability is almost always stronger in the bluer
passband than in the redder, the variability ratio depends on whether
strong emission lines contribute to either one band or the other. We find
that the observed variability ratio-redshift relations are described well
assuming that (a) the r.m.s. fluctuation of the quasar continuum flux
follows a power law {sigma} (f_{lambda}) is proportional to {lambda}^-2^
(i.e., is bluer when brighter) and (b) the variability of the emission
line flux is only ~10% of that of the underlying continuum. These results,
based upon the photometry of more than 8000 quasars, confirm the previous
findings by Wilhite and collaborators for 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 the opposite of that expected from the dilution effect of
variability due to line emission and may be indicative of high Eddington
ratios in these unusual quasars. 

Key words: galaxies: active, quasars: general, quasars: emission lines