Ultraviolet Continuum Color Variability of Luminous Sloan Digital Sky
  Survey QSOs
  We examine whether the spectral energy distribution of UV continuum emission
of active galactic nuclei changes during flux variation. We used multi-epoch
photometric data of QSOs in the Stripe 82 observed by the SDSS Legacy Survey
and selected 10 bright QSOs observed with high photometric accuracies, in the
redshift range of z = 1.0-2.4 where strong broad emission lines such as
Ly\alpha and CIV do not contaminate SDSS filters, to examine spectral variation
of the UV continuum emission with broadband photometries. All target QSOs
showed clear flux variations during the monitoring period 1998-2007, and the
multi-epoch flux data in two different bands obtained on the same night showed
a linear flux-to-flux relationship for all target QSOs. Assigning the flux in
the longer wavelength to the x-axis in the flux-to-flux diagram, the
x-intercept of the best-fit linear regression line was positive for most
targets, which means that their colors in the observing bands become bluer as
they become brighter. Then, the host-galaxy flux was estimated on the basis of
the correlation between the stellar mass of the bulge of the host galaxy and
the central black hole mass. We found that the longer-wavelength flux of the
host galaxy was systematically smaller than that of the fainter extension of
the best-fit regression line at the same shorter-wavelength flux for most
targets. This result strongly indicates that the spectral shape of the
continuum emission of QSOs in the UV region usually becomes bluer as it becomes
brighter. We found that the multi-epoch flux-to-flux plots could be fitted well
with the standard accretion disk model changing the mass accretion rate with a
constant black hole mass for most targets. This finding strongly supports the
standard accretion disk model for UV continuum emission of QSOs.
