  Quasars are variable on timescales from days to years in UV/optical, and
generally appear bluer while they brighten. The physics behind the variations
in fluxes and colors remains unclear. Using SDSS g and r band photometric
monitoring data of quasars in Stripe 82, we find that although the flux
variation amplitude increases with timescale, the color variability exhibits
opposite behavior. The color variability of quasars is prominent at timescales
as short as ~ 10 days, but gradually reduces toward timescales up to years. In
other words, the variable emission at shorter timescales is bluer than that at
longer timescales. This timescale dependence is clearly and consistently
detected at all redshifts from z = 0 to 3.5, thus can not be due to
contaminations to broadband photometry from emission lines which do not respond
to fast continuum variations. The discovery directly rules out the possibility
that simply attributes the color variability to contamination from a
non-variable redder component, such as the host galaxy. It can not be
interpreted as changes in global accretion rate either. The thermal accretion
disk fluctuation model is favored, in the sense that fluctuations in the inner
hotter region of the disk are responsible for short term variations, while
longer term and stronger variations are expected from larger and cooler disk
region. An interesting implication is that one can use quasar variations at
different timescales to probe disk emission at different radii.

Astrophysics of Galaxies