M31N 2013-10c: A Newly Identified Recurrent Nova in M31

The nova M31N 2023-11f (2023yoa) has been recently identified as the second eruption of a previously recognized nova, M31N 2013-10c, establishing the latter object as the 21st recurrent nova system thus far identified in M31. Here we present well sampled $R$-band lightcurves of both the 2013 and 2023 eruptions of this system. The photometric evolution of each eruption was quite similar as expected for the same progenitor system. The 2013 and 2023 eruptions each reached peak magnitudes just brighter than $R\sim16$, with fits to the declining branches of the eruptions yielding times to decline by two magnitudes of $t_2(R)=5.5\pm1.7$ and $t_2(R)=3.4\pm1.5$ days, respectively. M31N 2013-10c has an absolute magnitude at peak, $M_R=-8.8\pm0.2$, making it the most luminous known recurrent nova in M31.


INTRODUCTION
A recent nova in the Andromeda galaxy, M31N 2023-11f, has been confirmed by Shafter et al. (2023) to be the second known eruption of a previously identified nova M31N 2013-10c (Hornoch et al. 2013a,b).The identification of M31N 2013-10c as a recurrent nova (RN) brings to 21 the total number of such systems known in M31 -more than twice the number of RNe known in the Milky Way (Schaefer 2010).The observed time interval between eruptions establishes an upper limit of ∼10.1 yr on the recurrence time (it is conceivable, though unlikely, that one or more eruptions may have been missed).Such a short recurrence time is typical of RNe in M31, but is quite short by Galactic standards where of the 10 RNe known, only U Sco (t recur ≃ 10.3 yr) has a comparably short recurrence interval.Here, we present R-band lightcurves for the two known eruptions of M31N 2013-10c, demonstrating that, as expected, they are quite similar.

THE LIGHT CURVES
M31N 2013-10c was discovered by the Lick Observatory Supernova Search (LOSS) on 2013 October 09.30UT at a magnitude, m = 18.8 (unfiltered).Shortly thereafter, an observation by N. James on October 09.813 UT revealed that the object had brightened to m = 15.9 (unfiltered) 1 .A series of R-band measurements were subsequently made with the 0.65-m reflector at the Ondřejov Observatory.The top panel of Figure 1 shows the full lightcurve for M31N 2013-10c.The declining portion of the light curve for both eruptions is best represented by a two-component linear fit (shown as broken lines in Figure 1).The fit to the M31N 2013-10c data suggests t 2 (R) = 5.A1.
M31N 2023-11f was discovered by Xu et al. (2023) at m = 16.2 (unfiltered) on 2023 November 27.831 UT.A subsequent measurement made on 2023 November 28.130 UT showed that the nova had brightened to R = 15.79±0.04,which we take as maximum light.A series of R-band measurements over the next 2 weeks were made at the Mount Laguna, Ondřejov, and La Silla observatories.These observations were supplemented by Sloan r measurements taken at the Xinjiang Observatory, which have been converted to R using the transformations given in Jordi et al. (2006).The resulting lightcurve is presented in the middle panel of Figure 1.The corresponding fit to the declining portion of the lightcurve (dotted line) yields t 2 (R) = 3.4 ± 1.5 d.
The bottom panel shows the two eruptions superimposed with the abscissa showing the time relative to that of peak brightness for each eruption.It is clear that the lightcurves of the two eruptions are similar, with peak observed magnitudes of m = 15.9 (unfiltered) and R = 15.79 for the 2013 and 2023 eruptions, respectively, and rates of decline that are also similar.A two-component fit of the combined lightcurve data yields a mean time to decline 2 mag from maximum light, ⟨t 2 (R)⟩ = 3.9 ± 1.0 d.
At the distance of M31 (µ 0 = 24.42 ± 0.06, Tully et al. 2013), and adopting a foreground R-band extinction of 0.14 mag (Schlafly & Finkbeiner 2011), we find that M31N 2013-10c reaches an absolute magnitude, M R ≃ −8.8 ± 0.2, where we have estimated a generous uncertainty taking into account the possibility that maximum light might have been missed.

DISCUSSION
Models show that a relatively high rate of accretion ( Ṁ ∼ > 10 −7 M ⊙ yr −1 ) onto a massive white dwarf (M W D ∼ > 1.3 M ⊙ ) produce relatively weak nova eruptions that recur frequently (t recur ∼ < 100 yr) an eject a relatively small amount of mass (e.g., see Kato et al. 2014).The small ejected mass results in a relatively rapid photometric evolution, with t 2 times typically under 10 days.These properties result in an M31 RN population that is characterized by the following median values ⟨t recur ⟩ = 9.6 yr, ⟨M R ⟩ = −7.0,and ⟨t 2 ⟩ = 6.0 d.Clearly, M31N 2013-10c is not atypical in terms of its recurrence time and rate of decline, but is significantly more luminous than the typical M31 RN system.
Ongoing monitoring of M31 will confirm whether the recurrence time of M31N 2013-10c is indeed ∼ 10 yr, or whether it might be an integer fraction of that value.Finally, 10 of the 13 M31 RNe that have been spectroscopically confirmed are members of the He/N class (e.g., see Williams 1992, for a discussion of spectroscopic classifications).Thus, it will also be important to obtain spectroscopy during the next eruption to establish the spectroscopic class of this unusually luminous RN.

Figure 1 .
Figure 1.The lightcurves for the 2013 October (M31N 2013-10c, top panel) and 2023 November (M31N 2023-11f, middle panel) eruptions of the RN M31N 2013-10c.The bottom panel shows both eruptions superimposed after adopting times of maximum of JD2,456,575.313and 2,460,276.630for the 2013 and 2023 eruptions, respectively.The star and the open square in the upper panel corresponds to the photometry reported by the LOSS and N. James, respectively, while the open square in the middle panel shows the unfiltered discovery magnitude ofXu et al. (2023).The open circles in the middle panel represent Sloan r photometry converted to Cousins R as described in the text.The downward-facing triangles represent lower limits from prediscovery images.The dashed line in the lower panel represents the best-fitting two-component line used to estimate ⟨t2(R)⟩ = 3.9 ± 1.0 d.The photometric data are given in TableA1.