Confirmation of AT2021vgi (= M31N 2021-08a) as the fourth observed eruption of the recurrent nova M31N 1997-11k

ATel #14862; A. W. Shafter (SDSU), K. Hornoch (Astronomical Institute, Ondrejov, Czech Republic), H. Kucakova (Ondrejov)
on 14 Aug 2021; 04:53 UT
Credential Certification: Allen W. Shafter (ashafter@sdsu.edu)

Tweet

A. Valcheva et al. (ATel #14842, ATel #14853) recently reported that the position of the M31 transient AT2021vgi was within 0.45" of the known M31 nova M31N 2009-11b. To confirm the association between these two objects, we have obtained direct imaging of the field and found that the objects are spatially coincident to better that 0.4" (see the link to the comparison image below). The same instrumentation was used in acquiring both images (0.65-m at Ondrejov + CCD + R filter), with the M31N 2009-11b image taken on 2009 Nov. 26.779 UT and the M31N 2021-08a image taken on 2021 Aug. 12.017 UT. Since, as shown by Shafter et al. (2015, ApJS, 216, 34), M31N 2009-11b was the second observed recurrence of the known M31 recurrent nova M31N 1997-11k (the other recurrence being M31N 2001-12b), we confirm that AT2021vgi (= M31N 2021-08a) is the third observed recurrence (fourth observed eruption) of M31N 1997-11k.

Given that the three recurrences have occurred ~4, ~12, and ~24 years after the first observed eruption, it seems likely that the recurrence period of M31N 1997-11k is ~4 years. The recurrent nova is relatively faint (typically reaching a peak magnitude of just ~18.5), which may explain how predicted eruptions in 2013 and 2017 could have been missed. However, unlike the rapidly-recurring M31 nova M31N 2008-12a (Darnley et al. 2014, A&A, 563, 9; Tang et al. 2014, ApJ, 786, 61), M31N 1997-11k displays a relatively slow light curve evolution (t_2 ~ 90 d; Shafter et al. 2011, 734, 12), which should increase its likelihood of detection. Given its potentially very short recurrence time, M31N 1997-11k should be carefully monitored over the coming years to assure that future eruptions are not missed.

Comparison of M31N 2009-11b (black) and M31N 2021-08a (white)