Microlens diagnostics of accretion disks in active galactic nuclei

The optical-ultraviolet continuum from active galactic nuclei (AGN) seems t o originate from optically thick and/or thin disks, and occasionally from a ssociated circumnuclear starburst regions. These different possible origins can, in principle, be distinguished by observations of gravitational micro lensing events. We performed numerical simulations of microlensing of an AG N disk by a single lensing star passing in front of the AGN. Calculated spe ctral variations and light curves show distinct behavior, depending on the nature of the emitting region; time variation over few months with strong w avelength dependence is expected in the case of an optically thick disk (st andard disk), while an optically thin disk (advection-dominated disk) will produce shorter, nearly wavelength-independent variation. In the case of an associated circumnuclear starburst region much slower variations (over a y ear) will be superposed on the shorter variations caused by microlensing of the disk.