Independent Discovery and Photometry of a Very Slowly Evolving Nova in M31
ATel #15831; K. Hornoch, H. Kucakova (Astronomical Institute, Ondrejov, Czech Republic), A. W. Shafter, J. Della Costa, J. C. Horst, J. G. Clark, W. A. Burris, R. M. Quimby, J. Bardwell, A. Choudhary, JT Markham-Adkison, C. Martin, P. Spalding, A. Tedeschi, M. Thornton, J. Valladon (SDSU)
on 28 Dec 2022; 22:38 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Allen W. Shafter (ashafter@sdsu.edu)
Subjects: Optical, Nova, Transient
Referred to by ATel #: 15926
We report the independent discovery of very slowly evolving nova during our survey of the central region of M31. We detected the object for the first time on an average R-band CCD frame taken on 2022 Sep. 25.237 UT with the 40-in reflector at SDSU's Mount Laguna Observatory. Since our initial discovery, we have continued to follow the object photometrically in order to fully characterize the lightcurve.
The object was independently discovered on Nov. 1.621 UT by M. Zhang et al. and designated as PNV J00423788+4112491 = AT 2022abzg. The object has been designated by us as M31N 2022-09b, and is located at R.A. = 0h42m37s.90, Decl. = +41o12'49".2 (equinox 2000.0), which is 72.5" west and 199.3" south of the center of M31 (see link to a finding chart below).
As shown in the Table below, which is taken from our extensive photometry (to be published separately), the object brightened very slowly (~0.08 mag/day) for at least 2 months until it reached a maximum at R ~ 17.5 mag on 2022 Dec. 8 UT. Such a gradual brightening is unusual for a nova, but typical of slowly varying Mira variables in M31 that can sometimes masquerade as novae. Given the initial uncertainty surrounding the object, we postponed any discovery announcement until we could be quite confident that the transient was indeed a nova.
Since novae develop strong Balmer emission post maximum light, it is possible to distinguish a nova from other types of variable sources (e.g. Mira variables) photometrically by imaging the nova field through both narrow-band Hα and broad-band R filters. Normal stars and Mira variables will have similar R and Hα flux densities, while novae will show pronounced Hα excesses on the declining branches of their eruptions. Thus, once the transient reached maximum light and started to fade, we augmented our extensive R-band observations with occasional Hα (FWHM = 70A) images.
The following photometric measurements were made using the 1.0-m telescope at Mount Laguna Observatory (MLO), the 1.54-m Danish telescope at La Silla Observatory (DK154), and the 0.65-m telescope at Ondrejov (OND):
Date [UT] | Mag | Err | Filt | Telescope
09 24.240 | <21.50 | | R | MLO
09 25.237 | 21.80 | 0.60 | R | MLO
09 26.232 | 21.30 | 0.40 | R | MLO
10 02.319 | 20.40 | 0.30 | R | MLO
10 25.208 | 20.00 | 0.25 | R | MLO
10 26.158 | 20.20 | 0.25 | R | MLO
11 11.190 | 20.10 | 0.25 | R | MLO
11 19.113 | 19.20 | 0.15 | R | MLO
11 26.018 | 18.75 | 0.09 | R | DK154
12 04.026 | 18.60 | 0.06 | R | DK154
12 08.172 | 17.51 | 0.03 | R | MLO
12 10.119 | 17.83 | 0.06 | R | MLO
12 10.080 | 17.53 | 0.11 | Ha | MLO
12 16.000 | 18.44 | 0.05 | R | DK154
12 17.026 | 18.60 | 0.10 | R | DK154
12 17.069 | 16.70 | 0.10 | Ha | MLO
12 17.111 | 18.80 | 0.10 | R | MLO
12 19.090 | 18.80 | 0.15 | R | MLO
12 19.118 | 16.50 | 0.10 | Ha | MLO
12 19.221 | 18.62 | 0.09 | R | MLO
12 25.691 | 18.90 | 0.20 | R | OND
12 27.090 | 19.00 | 0.10 | R | MLO
As expected for a nova, the object did not show significant Hα emission near peak brightness, but this changed markedly as the object began to fade. Specifically, we measured Hα excesses relative to R-band of ~2.0 and ~2.2 mag on 2022 Dec. 17 and 19 UT, respectively. These values are typical of novae on the declining branch of an eruption.
Based on these findings we conclude that M31N 2022-09b is almost certainly a very slowly evolving classical nova in M31. As with other very slow novae, we wouldn't be surprised if this nova has a complex lightcurve with multiple re-brightenings, so follow-up photometric and spectroscopic observations of this very unusual nova are strongly encouraged.
Finding chart
