Sudden NIR brightening of the Andromeda galaxy Red Nova M31LRN 2015

ATel #7624; Peter Pessev, Stefan Geier (IAC, ULL, GRANTECAN, Spain), Maximilian Stritzinger (Aarhus University, Denmark), Alexander Kurtenkov (Sofia University, Bulgaria), Toma Tomov (Nicolaus Copernicus University, Torun, Poland)
on 12 Jun 2015; 12:16 UT
Credential Certification: Toma Tomov (tomov@umk.pl)

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We report a sudden brightening of the stellar merger candidate MASTER OT J004207.99+405501.1 / M31N 2015-01a / M31LRN 2015 (Kurtenkov et al. 2015, Williams et al. 2015, ATel #6911, ATel #7150) in the NIR part of the spectrum. Our observations were carried out with the 2.5m Nordic Optical Telescope (NOT) at the Observatorio Roque de Los Muchachos (ORM), La Palma, utilizing the NOTCam instrument. The observations were executed in the morning hours of May 30th, June 01st and June 02nd. Note that on June 02nd one quadrant of the NOTCam detector went out of order, but the observer applied necessary measures to mitigate the effect on the observations. We also took special precautions when analyzing these data. Five nearby 2MASS stars with accurate magnitudes (AAA quality flags) were used for the photometric calibration. The photometry data points are listed below:

UT Band Mag Merr AM Seeing

2015-05-30.21 Ks 14.75 0.06 1.65 0.95"

2015-06-01.23 J 17.79 0.09 1.50 0.77"

2015-06-01.23 H 16.31 0.07 1.47 0.80"

2015-06-01.23 Ks 14.74 0.06 1.45 0.75"

2015-06-02.22 J 17.34 0.12 1.56 0.82"

2015-06-02.22 H 15.60 0.04 1.53 0.75"

2015-06-02.22 Ks 14.03 0.07 1.50 0.73"

Note that our Ks-band May 30th measurement is in very good agreement with the results reported in ATel #7595 - 14.7+/-0.1 versus 14.75+/-0.06. Between June 01st and June 02nd we observe a significant brightening of the NIR emission from the transient, $\Delta$J=0.45 +/- 0.15 mag, $\Delta$H = 0.71 +/- 0.08 mag, $\Delta$Ks=0.71 +/- 0.09 mag. The observed change of the colors of the object is summarized below:

UT J-H H-Ks J-Ks

2015-06-01.23 1.48 +/- 0.11 1.57 +/- 0.09 3.05 +/- 0.11

2015-06-02.22 1.74 +/- 0.13 1.57 +/- 0.08 3.31 +/- 0.14

$\Delta$ 0.26 +/- 0.17 0.00 +/- 0.12 0.26 +/- 0.18

Taking into account the reddening estimates reported by Kurtenkov et al. 2015 and the intrinsic characteristics of the stars tabulated by Ducati et al. 2001, at the moment of the reported observations the transient shows NIR colors redder than those expected even for the most evolved stellar objects.

More observations are required to pinpoint whether we are observing a general brightening of the transient, similar to those reported for V838 Mon, or whether dust formation is shifting the colors and the maximum of the SED to the red.