Probable M31 Stellar Merger - NOT ALFOSC Photometry

ATel #7272; P. Pessev, S. Geier (ULL, IAC, GRANTECAN), A. Kurtenkov (University of Sofia, Bulgaria), L. D. Nielsen (NOT), T. Tomov (Uniwersytet Mikolaja Kopernika, Torun, Poland)
on 22 Mar 2015; 22:39 UT
Credential Certification: Toma Tomov (tomtom@astri.uni.torun.pl)

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Observations of the probable stellar merger ("Nova" M31N 2015-01a = MASTER J004207.99+405501.1 - ATel #6911, #6924, #6941, #6951, #6952, #6985, #7150, #7208, #7236) are reported. Two epochs of optical BRI Johnson-Cousins photometry were obtained on 2015 March ~ 11.84 UT and March ~ 14.85 UT with the ALFOSC instrument on the 2.5m Nordic Optical Telescope (NOT). These observations were carried out at the end of the astronomical twilight at a high airmass, and were obtained close to the limit of the visibility of the object from the Observatorio Roque de los Muchachos (ORM). To mitigate the effects of the atmospheric extinction and possible systematics on the photometry, nearby reference stars calibrated with the Massey (2006) catalog were used as internal standards. The photometry data points are listed below:

UT Band Mag Err

2015-03-11.84 B 20.06 0.05

2015-03-11.84 R 17.01 0.01

2015-03-11.85 I 15.56 0.02

2015-03-14.85 B 20.38 0.05

2015-03-14.85 R 17.32 0.01

2015-03-14.86 I 15.74 0.02

Utilizing the Schlafly and Finkbeiner (2011) extinction estimate for the direction of M31 and the Fitzpatrick (1999) reddening law with Rv = 3.1, results to extinction values A = 0.225, 0.134, 0.093 respectively in the B, R, I photometric bands. Therefore the resulting extinction corrected colors (not taking into account the internal M31 reddening) are:

UT Color Mag Err

2015-03-11 B-R 2.96 0.05

2015-03-11 R-I 1.40 0.02

2015-03-14 B-R 2.97 0.05

2015-03-14 R-I 1.54 0.02

Comparing to results reported in (ATel #6985 and #7150), the initial rapid evolution of the B-R color had slowed down. Only a marginal increase within the error bars is present, comparing the results from 2015 February 21 UT and March 14 UT - from 2.84 +/- 0.10 to 2.97 +/- 0.05.

According to the intrinsic colors tabulated by Ducati et al. (2011), the R-I color is consistent with an early M giant (~ M4, see their table 4) , in agreement with the spectroscopic results reported in ATel #7208 and NIR colors from ATel #7236.

Considering that the object is not accessible from the major observatories in the Northern hemisphere any more, we strongly encourage further observations from smaller facilities within the zone of visibility of M31.