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The UV/optical variability of active galactic nuclei and quasars is usefulfor understanding the physics of the accretion disk and is gradually attributedto the stochastic fluctuations over the accretion disk. Quasars generallyappear bluer when they brighten in the UV/optical, the nature of which remainscontroversial. Recently \citeauthor{Sun2014} discovered that the colorvariation of quasars is timescale dependent, in the way that faster variationsare even bluer than longer term ones. While this discovery can directly ruleout models that simply attribute the color variation to contamination from thehost galaxies, or to changes in the global accretion rates, it favors thestochastic disk fluctuation model as fluctuations in the innermost hotter diskcould dominate the short-term variations. In this work, we show that a revisedinhomogeneous disk model, where the characteristic timescales of thermalfluctuations in the disk are radius-dependent (i.e., $\tau \sim r$; based onthe one originally proposed by \citeauthor{DexterAgol2011}), can well reproducea timescale dependent color variation pattern, similar to the observed one andunaffected by the un-even sampling and photometric error. This demonstratesthat one may statistically use variation emission at different timescales tospatially resolve the accretion disk in quasars, thus opens a new window toprobe and test the accretion disk physics in the era of time domain astronomy.Caveats of the current model, which ought to be addressed in futuresimulations, are discussed.

Simulating the timescale dependent color variation in quasars with a revised inhomogeneous disk model