AU-Scale Microlensing Diagnosis of Quasar Central Engine

The innermost region of quasars can be resolved by a gravitational lens ``telescope'' on scales down to a few AU. For this purpose, X-ray observations are most preferable, because X-rays originating from the innermost regions can be selectively amplified by microlensing due to the so-called `caustic crossing'. Light curves of such microlensing events are affected by the size and the structure of microlensed sources. Therefore, microlensing observations at X-rays reveal the detailed structure of an accretion disk in quasar. In this paper, we also show the result of simulation (e.g., microlensing light curve at wide wavelength ranges) using realistic accretion disk models (standard disk and advection-dominated accretion flow).