Signatures of Accretion Disks in Quasar Microlensing

We propose that relative variability on short time-scales of the multipleimages of a lensed quasar, after removal of the time delay, may be caused byhot spots or other moving structures in the accretion disk crossing microlenscaustics caused by stellar mass objects in the lensing galaxy. Such variabilityhas been reported in the two images of 0957+561. The short durations would bedue to the high rotation speed of the disk (v/c ~ 0.1), rather than planetarymass objects in the slowly moving (v/c ~ 0.001) lens. This interpretation couldbe confirmed by finding periodicity, or correlations of the spectral and fluxvariations due to the Doppler effect in the disk. We also propose anothersignature of stationary accretion disks (with no intrinsic variability): thegradient of the magnification over the accretion disk should cause a relativecolor change between the images whose sign and amplitude are correlated withthe time derivative of the flux difference between the images. Other colorterms induced by the radial variation of disk colors are of second order in themagnification gradient. The methods proposed here can be used first to verifythat accretion disks near supermassive black holes are the source of thecontinuum radiation from quasars, and then to study them.Comment: 11 pages plus 1 postscript figur