Direct imaging of the immediate environments of black holes in Active Galactic Nuclei (AGNs) requires angular resolutions of the order of tens of nano-seconds at z~1 and is beyond the capabilities of present day telescopes.
We have recently developed an indirect method to probe the various emission regions of an accretion disk from scales of a few hundred gravitational radii (r_g) down to the event horizon of the black hole. This method is based on monitoring microlensing events in lensed quasars. In particular, as a magnification caustic is traversing the accretion disk we expect to detect changes in the line energy, intensity, and profile of microlensed Fe Ka lines in the X-ray spectra of lensed quasars.
We reported the first detection of a such a microlensing event in the Chandra observations of the radio-loud quasar MG J0414+0534 (Chartas et al. 2002, ApJ, 568, 509). Based on our simulations of caustic crossings, we expect future detailed X-ray monitoring of microlensing events in quasars to reveal significant distortions of the iron line caused by special and general relativistic effects and Doppler effects as the caustic approaches near (a few r_g) the event horizon of the black hole. In a recent paper (Yonehara et al. 2003, ApJ, 594, 107) we proposed a method to map the locations of accretion disk flares in quasars. This technique will require observations of gravitationally lensed quasars in the X-ray band with telescopes having subarcsec resolution and collecting areas of the order of 10,000 cm^2 (e.g., XEUS). Our detection of rapid X-ray flares in several distant gravitationally lensed quasars indicates that our proposed method of mapping the spatial distribution of X-ray flares in quasar accretion disks will be feasible in the future.