  The variability of quasars across multiple wavelengths is a useful probe of
physical conditions in active galactic nuclei. In particular, variable
accretion rates, instabilities, and reverberation effects in the accretion disk
of a supermassive black hole (SMBH) are expected to produce correlated flux
variations in UV and optical bands. Recent work has further argued that binary
quasars should exhibit strongly correlated UV and optical periodicities. Strong
UV-optical correlations have indeed been established in small samples of up to
approximately 30 quasars with well-sampled light curves, and have extended the
"bluer-when-brighter" trend previously found within the optical bands. Here we
further test the nature of quasar variability by examining the observed-frame
UV-optical correlations in a large sample of 1,315 bright quasars with
overlapping UV and optical light curves for the Galaxy Evolution Explorer
(GALEX) and the Catalina Real-time Transient Survey (CRTS), respectively. We
find that strong correlations exist in this much larger sample, but we rule
out, at approximately 95% confidence, the simple hypothesis that the intrinsic
UV and optical variations of all quasars are fully correlated. Our results
therefore imply the existence of physical mechanism(s) that can generate
uncorrelated optical and UV flux variations.
