J004457+4123 (Sharov 21): not a remarkable nova in M31 but a background quasar with a spectacular UV flare

TL;DR

A background quasar behind M31 was mistaken for a nova; its UV flare is likely due to a stellar tidal disruption event or gravitational microlensing, with detailed long-term observations supporting this interpretation.

Contribution

This study reclassifies Sharov 21 as a quasar with a rare UV flare, providing extensive photometric data and analysis of the flare's origin, challenging previous nova classification.

Findings

The UV flare was about 20 times brighter than typical quasar variability.

The flare's energy exceeds that of luminous supernovae, suggesting an extreme event.

The flare profile matches predictions for a stellar tidal disruption event.

Abstract

We announce the discovery of a quasar behind the disk of M31, which was previously classified as a remarkable nova in our neighbour galaxy. The paper is primarily aimed at the outburst of J004457+4123 (Sharov 21), with the first part focussed on the optical spectroscopy and the improvement in the photometric database. Both the optical spectrum and the broad band spectral energy distribution of Sharov 21 are shown to be very similar to that of normal, radio-quiet type 1 quasars. We present photometric data covering more than a century and resulting in a long-term light curve that is densely sampled over the past five decades. The variability of the quasar is characterized by a ground state with typical fluctuation amplitudes of ~0.2 mag around B~20.5, superimposed by a singular flare of ~2 yr duration (observer frame) with the maximum at 1992.81 where the UV flux has increased by a factor of ~20. The total energy in the flare is at least three orders of magnitudes higher than the radiated energy of the most luminous supernovae, provided that it comes from an intrinsic process and the energy is radiated isotropically. The profile of the flare light curve appears to be in agreement with the standard predictions for a stellar tidal disruption event where a ~10 M_sun giant star was shredded in the tidal field of a ~2...5 10^8 M_sun black hole. The short fallback time derived from the light curve requires an ultra-close encounter where the pericentre of the stellar orbit is deep within the tidal disruption radius. Gravitational microlensing provides an alternative explanation, though the probability of such a high amplification event is very low.