Through Andromeda and Beyond: AGN Optical Transient 'Sharov21' Revisited
  We revisit a notable AGN known as `Sharov21', seen to undergo a dramatic
outburst in 1992, brightening by a factor of thirty over a period of
approximately one year. A simple microlensing model fit to the event lightcurve
provides a constraint on the distance of the lensing object which is consistent
with the distance to M31, strongly suggesting that this is the correct
explanation. Archival XMM/Hubble/Spitzer data show that this AGN can be
considered an otherwise unremarkable type-I AGN. Our analysis of the expected
rate of background AGN being microlensed by a factor of two or more due to
stellar-mass objects in M31 shows that events of this nature should only occur
on average every half century. It is thus perhaps surprising that we have
uncovered evidence for two more events that are qualitatively similar. A
systematic search for new and archival events, with follow-up spectroscopy, is
thus warranted.
