The X-Ray Variability of a Large, Serendipitous Sample of Spectroscopic
  Quasars
  We analyze the X-ray variability of 264 Sloan Digital Sky Survey
spectroscopic quasars using the Chandra public archive. This data set consists
of quasars with spectroscopic redshifts out to z~5 and covers rest-frame time
scales up to Delta t_sys 2000 d, with 3 or more X-ray observations available
for 82 quasars. It therefore samples longer time scales and higher luminosities
than previous large-scale analyses of AGN variablity. We find significant (>3
sigma) variation in ~30% of the quasars overall; the fraction of sources with
detected variability increases strongly with the number of available source
counts up to ~70% for sources with >1000 counts per epoch. Assuming the
distribution of fractional variation is Gaussian, its standard deviation is
~16% on >1 week time scales, which is not enough to explain the observed
scatter in quasar X-ray-to-optical flux ratios as due to variability alone. We
find no evidence in our sample that quasars are more variable at higher
redshifts (z > 2), as has been suggested in previous studies. Quasar X-ray
spectra vary similarly to some local Seyfert AGN in that they steepen as they
brighten, with evidence for a constant, hard spectral component that is more
prominent in fainter stages. We identify one highly-variable Narrow Line
Seyfert 1-type spectroscopic quasar in the Chandra Deep Field-North. We
constrain the rate of kilosecond-timescale flares in the quasar population
using ~8 months of total exposure and also constrain the distribution of
variation amplitudes between exposures; extreme changes (>100%) are quite rare,
while variation at the 25% level occurs in <25% of observations. [OIII] 5007A
emission may be stronger in sources with lower levels of X-ray variability; if
confirmed, this would represent an additional link between small-scale (corona)
and large-scale (narrow line region) AGN properties.
