Through Andromeda and Beyond: AGN Optical Transient 'Sharov21' Revisited

TL;DR

This paper revisits the 1992 outburst of the AGN 'Sharov21', suggesting microlensing by M31 objects as the cause, and finds evidence for additional similar events, indicating such phenomena may be more common than previously thought.

Contribution

The study provides a detailed analysis of 'Sharov21's' outburst, supports microlensing as the cause, and uncovers potential multiple similar events, highlighting the need for systematic searches.

Findings

Microlensing explains 'Sharov21's' outburst consistent with M31 distance.

Expected rate of such microlensing events is about once every fifty years.

Evidence for two additional similar events suggests higher occurrence frequency.

Abstract

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.