M31N 2015-01a - A Luminous Red Nova

ATel #7150; A. Kurtenkov (University of Sofia, Bulgaria), T. Tomov (Uniwersytet Mikolaja Kopernika, Torun, Poland), S. Fabrika, E. A. Barsukova, A. F. Valeev (Special Astrophysical Observatory), P. Pessev (GRANTECAN, IAC), K. Vida, L. Molnar, K. Sarneczky (Konkoly Observatory, Hungary), V. P. Goranskij (SAI, Moscow University), K. Hornoch (Astronomical Institute, Ondrejov), M. Henze (ESA/ESAC, Spain), A. W. Shafter (SDSU), E. Ovcharov, P. Nedialkov (University of Sofia, Bulgaria), A. Kostov (IA BAS, Bulgaria), S. Valenti (LCOGT), M. Stritzinger (Aarhus University)
on 27 Feb 2015; 10:31 UT
Credential Certification: Toma Tomov (tomtom@astri.uni.torun.pl)

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Preliminary results of the ongoing optical monitoring of M31N 2015-01a = MASTER J004207.99+405501.1 (ATel #6911, #6924, #6941, #6951, #6952, #6985) are reported. Throughout February the colors of the object have been consistently shifting to the red. In just one month B-R has changed from 0.8 to 2.9 mag. The following magnitudes are reported:

date,UT filter mag err telescope

2015-01-24.11 B 16.00 0.04 McDonald 1 m, Fort Davis, Texas

2015-01-24.12 R 15.24 0.02 McDonald 1 m, Fort Davis, Texas

2015-02-07.74 B 17.86 0.05 Konkoly 60/90 cm Schmidt, Hungary

2015-02-07.74 R 15.74 0.03 Konkoly 60/90 cm Schmidt, Hungary

2015-02-18.7 B 18.48 0.12 Konkoly 60/90 cm Schmidt, Hungary

2015-02-18.7 V 16.87 0.04 Konkoly 60/90 cm Schmidt, Hungary

2015-02-18.7 R 15.90 0.03 Konkoly 60/90 cm Schmidt, Hungary

2015-02-18.7 I 15.22 0.03 Konkoly 60/90 cm Schmidt, Hungary

2015-02-21.72 B 18.77 0.09 6 m BTA, Russia

2015-02-21.72 V 16.84 0.03 6 m BTA, Russia

2015-02-21.72 R 15.84 0.03 6 m BTA, Russia

Additional spectroscopic observations were performed on Feb 21.79 and Feb 22.71 UT with the Russian 6m BTA telescope, equipped with the SCORPIO spectrograph. The spectral ranges were 3720 - 5530 Å and 5750 - 7500 Å with a resolution of about 5 Å. The spectrum shows a strong continuum with multiple deep absorption lines. The only emission observed is the Ca II doublet 3933 and 3968 Å.

Spectra with the 10.4m Gran Telescopio Canarias and the OSIRIS instrument were obtained on Feb 24. The spectral range of 5000-10000 Å was covered by three individual setups utilizing the R2500V, R and I grisms. The spectrum is dominated by absorption lines with weaker molecular bands (mainly TiO). The only emission seen in the OSIRIS spectra is weak Hα with a central absorption component.

The object displayed an unusual absorption-line F5I type spectrum in January 2015 (ATel #6985). During the February 2015 spectroscopic observations the object shows spectral features typical for a K supergiant. Furthermore, distinct similarities are found to the V838 Mon spectra taken in the second part of March - beginning of April 2002. Based on the extreme luminosity at maximum (L>3x10^5 Lsun), the initial slow decline, observed shift of colors, and spectral information we conclude that the object is a Luminous Red Nova (LRN).

This red nova may be entering a stage of rapid and deep fading. Taking into account the limited visibility of M31, we strongly encourage follow-up observations be made as soon as possible.