AT2022acwk is a Nova in M31

ATel #15863; Zaznobin I. A., Mereminskiy I. A., Burenin R. A., Sazonov S. Yu., Lutovinov A. A., Semena A. N. (IKI RAS, Moscow, Russia), Eselevich M. V. (ISTP, SB RAS, Irkutsk, Russia)
on 17 Jan 2023; 08:19 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)

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AT2022acwk/Gaia23afx is a new optical transient, discovered by ATLAS (Tonry et al., 2018) on December 12, 2022 with the magnitude of 16.4 in the orange o-filter and located 55' away from the center of M31. Gaia RP low-resolution spectrum obtained on January, 11, 2023 showed signatures of strong emission lines. Recent Gaia G-band photometry also suggests a significant decay of an optical flux, which is now at the level of 18.8 mag.

We observed the source with the 1.6-m telescope AZT-33IK of the Sayan observatory (Mondy) on January 16, 2023. An optical spectrum was obtained under reasonable conditions (seeing 3"-4"), with the ADAM spectrometer in a long-slit mode (2" slit width), covering a 3700-7250AA range. Total exposure was 1800s. The continuum is barely seen, the spectrum is dominated by broad, prominent Balmer lines (Hα - Hγ) at a blueshift of ~0.001, consistent with the M31 radial velocity. The Hα and Hβ FWHM velocities are ~1400 km/s and ~1300 km/s, correspondingly. Weak HeI lines at 5875AA and 7065AA (restframe) are also seen.

Given the spectral properties and a peak absolute magnitude of about -8 we classify AT2022acwk as a new Nova in the outer part of the M31 stellar disk.