AN X-RAY MICROLENSING TEST OF AU-SCALE ACCRETION DISK STRUCTURE IN Q2237+0305

The innermost regions of quasars can be resolved by a gravitational le ns ''telescope'' on scales down to a few AU. For this purpose, X-ray o bservations are most preferable because X-rays originating from the in nermost regions can be selectively amplified by microlensing resulting from the ''caustic crossing.'' If detected, X-ray variations will con strain the size of the X-ray-emitting region down to a few AU. The max imum attainable resolution depends mainly on the monitoring intervals of lens events, which should be much shorter than the crossing time. O n the basis of this idea, we performed numerical simulations of microl ensing of an optically thick, standard-type disk as well as an optical ly thin, advection-dominated accretion flow (ADAF). Calculated spectra l variations and light curves show distinct behaviors, depending on th e photon energy. X-ray radiation that is produced in optically thin re gion exhibits intensity variation over a few tens of days. In contrast , optical-UV fluxes, which are likely to come from optically thick reg ion, exhibit more gradual light changes, which is consistent with the microlensing events so far observed in Q2237+0305. Currently, 02237+03 05 is being monitored in the optical range at Apache Point Observatory . Simultaneous multiwavelength observations by Xray satellites (e.g., ASCA, AXAF, XMM) as well as HST at the moment of a microlens event ena ble us to reveal an AU-scale structure of the central accretion disk a round a black hole.