M31N 2022-11b: Additional Photometry and Confirmation as the Fourth Reported Eruption of the M31 Recurrent Nova M31N 1984-07a

ATel #15779; A. W. Shafter, J. G. Clark, R. M. Quimby, J. Bardwell, A. Choudhary, JT Markham-Adkison, C. Martin, P. Spalding, A. Tedeschi, M. Thornton, J. Valladon (SDSU), K. Hornoch, H. Kucakova (Astronomical Institute, Ondrejov, Czech Republic)
on 27 Nov 2022; 20:19 UT
Credential Certification: Allen W. Shafter (ashafter@sdsu.edu)

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As reported recently in ATel #15760, the optical transient M31N 2022-11b (= PNV J00424719+4116196, AT 2022zyr) was seen to erupt near the center of M31 (32.3" east and 11.3" north of the nucleus), and very close to the position of the known recurrent nova M31N 1984-07a (= 2004-11f, 2012-09a). Given the high density of novae this close to the nucleus, the possibility that the latest eruption was unrelated to the recurrent nova could not be immediately excluded.

In order to confirm the association of M31N 2022-11b with the recurrent nova M31N 1984-07a, we have carefully registered an image of M31N 2022-11b taken near maximum light with SDSU's Mount Laguna Observatory (MLO) 40-in reflector on 2022 Nov 13.112 UT with an image of M31N 2012-09a (the third recorded eruption of M31N 1984-07a) taken with the 2.54-m Isaac Newton Telescope on 2012 Sep 14.01 UT. The images of the nova are spatially coincident to better than 0.8" as shown in the registered comparison image linked below. Given the density of novae near the nucleus of M31, we estimate the probability that an unrelated nova would erupt within 0.8" of M31N 2012-09a over the past decade is only 0.0033. Thus, we conclude that M31N 2022-11b is the fourth reported eruption of the known recurrent nova M31N 1984-07a.

The timings of the four eruptions do not suggest a clear mean recurrence time for this nova. A recurrence time of ~7 years would fit the first three eruptions reasonably well, but not the last two which were seen ~10 years apart. An interval of ~10 years would fit the first two eruptions and the last two eruptions very well, but would overestimate the 7.8 yr interval between the second and third eruptions. Nevertheless, it appears that a mean recurrence time of order a decade best fits the available data.

Since the discovery of this latest eruption of M31N 1984-07a, we have been following the photometric evolution of M31N 2022-11b to characterize its light curve; and, in particular, to determine its rate of decline from maximum light (its t₂ time). As part of this effort, the following preliminary R-band magnitudes have been measured using the 40-in telescope at MLO, the 0.65-m telescope at Ondrejov (OND), and the 1.54-m Danish Telescope at La Silla (DK154):

 
   Date [UT]    |  Mag   | Err  | Filter | Telescope 
 
2022 Nov 10.953 | <19.5  |      |   R    | OND 
2022 Nov 11.188 | <19.2  |      |   R    | MLO 
2022 Nov 12.028 |  18.0  | 0.20 |   R    | OND 
2022 Nov 12.077 |  17.2  | 0.15 |   R    | OND 
2022 Nov 12.125 |  17.08 | 0.06 |   R    | MLO 
2022 Nov 12.271 |  16.56 | 0.05 |   R    | MLO 
2022 Nov 12.695 |  16.15 | 0.08 |   R    | OND 
2022 Nov 12.708 |  16.13 | 0.07 |   R    | OND 
2022 Nov 13.112 |  16.37 | 0.05 |   R    | MLO 
2022 Nov 14.181 |  16.92 | 0.07 |   R    | MLO 
2022 Nov 15.108 |  17.07 | 0.06 |   R    | MLO 
2022 Nov 16.139 |  17.36 | 0.09 |   R    | MLO 
2022 Nov 17.369 |  17.90 | 0.15 |   R    | MLO 
2022 Nov 19.115 |  18.20 | 0.20 |   R    | MLO 
2022 Nov 20.097 |  18.20 | 0.15 |   R    | MLO 
2022 Nov 21.013 |  17.89 | 0.07 |   R    | DK154 
2022 Nov 22.015 |  18.11 | 0.09 |   R    | DK154 
2022 Nov 22.117 |  18.40 | 0.20 |   R    | MLO 
2022 Nov 23.017 |  18.30 | 0.20 |   R    | DK154 
2022 Nov 23.122 |  18.60 | 0.20 |   R    | MLO 
2022 Nov 24.017 |  18.70 | 0.25 |   R    | DK154 
2022 Nov 25.018 |  18.50 | 0.20 |   R    | DK154 
2022 Nov 26.021 |  18.60 | 0.25 |   R    | DK154 
2022 Nov 26.104 |  18.80 | 0.25 |   R    | MLO 
 

For completeness, we have also included the first few measurements reported previously (ATel #15760). A weighted least squares fit to the declining branch of the light curve yields a preliminary t₂ time of 10.0±0.3 days, which is fast, but not unusually so for a nova with a mean recurrence time ~10 years or less.

Comparison Image