Photometry of a Probable Nova in M31 (2015-09a) on the Rise to or Near Peak Light

ATel #8048; G. S. Stringfellow, K. Mazur, E. Nelson, B. Bell, M. McKay, P. Wright (University of Colorado)
on 13 Sep 2015; 09:04 UT
Credential Certification: Guy S. Stringfellow (Guy.Stringfellow@Colorado.edu)

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We report optical photometry of a new probable nova in M31. Our ongoing program is surveying M31 for transients and is being conducted with the 0.5m Astrophysical Research Consortium Small Aperture Telescope (ARCSAT) located at Apache Point Observatory. The instrument used was SurveyCam with 0.93'' pixels when binned 2x2. The object was first reported to CBAT by Nishiyama and Kabashima on 2015 September 10.64 UT as TCP J00424034+4119295 with an unfiltered CCD magnitude of 18.5. The following night (2015 September 11.53 UT) they reported 17.5 mag, indicating the object had brightened by 1 mag. On 2015 September 12 UT we conducted a consecutive sequence of 3x300s images taken in BVR filters. The new object was detected in each individual image, though below we report the photometry from the median combined images in each filter:

2015 September 12.25 UT, B = 16.89 +/- 0.07
2015 September 12.26 UT, V = 16.77 +/- 0.06
2015 September 12.28 UT, R = 16.65 +/- 0.08

2015 September 08.45 UT, R > 17.70 (limiting magnitude)

The UT date listed coincides with midway between each filter exposure sequence. The object has continued to brighten by ~1 mag over the subsequent night. We did not detect the object in a similar combined R-image (3x300s) secured on 2015 September 08 UT, and the limiting magnitude at its location is reported above for that date. If indeed a nova, this places a constraint on the explosion having occurred between 2015 September 08.45 and 10.64 UT, and the nova was caught prior to reaching maximum light. Noguchi reported to CBAT an unfiltered CCD magnitude of 16.8 on 2015 September 12.50 UT, consistent with our filtered photometry secured shortly before on the same night. These last sets of observations are too close in time to definitively indicate that the object has yet reached peak light. Based on the several day rise time after discovery, this appears to be a slow nova. Our coordinates for the new object are RA=00h42m40.37s and Dec=+41o19'30.1'' based on registration with the combined R image in F4 of Massey et al. (2006, AJ, 131, 2478), which is offset by ~1" to the NE of the initial reported position.

Spectroscopic observations are needed to confirm this as a nova and to identify the nova class. Continued photometric monitoring is encouraged.