Stellar transits in active galactic nuclei
  Supermassive black holes (SMBH) are typically surrounded by a dense stellar
population in galactic nuclei. Stars crossing the line of site in active
galactic nuclei (AGN) produce a characteristic transit lightcurve, just like
extrasolar planets do when they transit their host star. We examine the
possibility of finding such AGN transits in deep optical, UV, and X-ray
surveys. We calculate transit lightcurves using the Novikov--Thorne thin
accretion disk model, including general relatistic effects. Based on the
expected properties of stellar cusps, we find that around 10^6 solar mass
SMBHs, transits of red giants are most common for stars on close orbits with
transit durations of a few weeks and orbital periods of a few years. We find
that detecting AGN transits requires repeated observations of thousands of low
mass AGNs to 1% photometric accuracy in optical, or ~ 10% in UV bands or soft
X-ray. It may be possible to identify stellar transits in the Pan-STARRS and
LSST optical and the eROSITA X-ray surveys. Such observations could be used to
constrain black hole mass, spin, inclination and accretion rate. Transit rates
and durations could give valuable information on the circumnuclear stellar
clusters as well. Transit lightcurves could be used to image accretion disks
with unprecedented resolution, allowing to resolve the SMBH silhouette in
distant AGNs.
