NIR monitoring of M31LRN2015

ATel #8059; 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 15 Sep 2015; 16:54 UT
Credential Certification: Toma Tomov (tomov@umk.pl)

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We report JHKs NIR photometry 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). 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 August 03rd and September 03rd. As outlined in ATel #7624, 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

2015-08-03.20 J 18.362 0.101 1.03 0.75"

2015-08-03.21 H 16.83 0.08 1.02 0.73"

2015-08-03.21 Ks 15.05 0.06 1.02 0.68"

2015-09-03.12 J 18.69 0.12 1.03 0.84"

2015-09-03.12 H 16.94 0.07 1.02 0.80"

2015-09-03.13 Ks 15.14 0.09 1.02 0.84"

The observed colors and change of the colors (compared to 2015-06-02UT) of the object is summarized below:

UT J-H H-Ks J-Ks

2015-08-03.21 1.52 +/- 0.13 1.78 +/- 0.10 3.31 +/- 0.12

2015-09-03.12 1.75 +/- 0.14 1.80 +/- 0.11 3.55 +/- 0.15

$\Delta$ 2015-08-03.21 0.22 +/- 0.18 -0.21 +/- 0.13 0.00 +/- 0.18

$\Delta$ 2015-09-03.12 0.01 +/- 0.19 0.23 +/- 0.14 0.24 +/- 0.21

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. We observe marginal variations (within the error bars) of the colors of the transient and a general increase of the NIR magnitudes. The rate in H band with respect of the 2015-06-02UT value is summarized below:

$\Delta H$ 2015-08-03.21 1.23 +/- 0.09

$\Delta H$ 2015-09-03.12 1.34 +/- 0.08

$\Delta H$ 0.11 +/- 0.11

More observations are required to further constrain the the photometric evolution of the transient.