The January 2015 outburst of a red nova in M31

TL;DR

This paper reports on the detailed optical observation and analysis of the January 2015 red nova in M31, providing insights into its lightcurve, spectral evolution, and luminosity, confirming its classification as a stellar merger event.

Contribution

It presents the first extensive optical monitoring of the 2015 M31 red nova, including high-precision lightcurve, spectral evolution, and luminosity estimates, expanding understanding of such rare stellar mergers.

Findings

Lightcurve duration of 70 days with accurate astrometry.

Spectral type evolved from F5I to K3I over 36 days.

Peak luminosity estimated at approximately 8.7 x 10^5 solar luminosities.

Abstract

M31N 2015-01a (or M31LRN 2015) is a red nova that erupted in January 2015 -- the first event of this kind observed in M31 since 1988. Very few similar events have been confirmed as of 2015. Most of them are considered to be products of stellar mergers. Results of an extensive optical monitoring of the transient in the period January-March 2015 are presented. Eight optical telescopes were used for imaging. Spectra were obtained on BTA, GTC and the Rozhen 2m telescope. We present a highly accurate 70 d lightcurve and astrometry with a 0.05" uncertainty. The color indices reached a minimum 2-3 d before peak brightness and rapidly increased afterwards. The spectral type changed from F5I to F0I in 6 d before the maximum and then to K3I in the next 30 d. The luminosity of the transient was estimated to $8.7^{+3.3}_{-2.2}\times10^{5}L_{\odot}$ during the optical maximum. Both the photometric and the spectroscopic results confirm that the object is a red nova, similar to V838 Monocerotis.