H-alpha observations of M31 novae AT2021zdh and AT2021ypn

ATel #14956; A. Valcheva (Sofia University, Bulgaria), M. Minev (Sofia University; IA with NAO, BAS, Bulgaria), I. Dimitrova (Sofia University; IA with NAO, BAS, Bulgaria), E. Ovcharov (Sofia Univeristy, Bulgaria), P. Nedialkov (Sofia University, Bulgaria)
on 6 Oct 2021; 08:46 UT
Credential Certification: Antoniya Valcheva (valcheva@phys.uni-sofia.bg)

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We report H-alpha photometry of two recent novae in M31 - AT2021zdh (= XM58MZ) (= PNV J00424123+4114322) and AT2021ypn (= PSP21H)(= PNV J00424297+4117574). The observations in narrowband H-alpha (FWHM of 30Å) were performed in the direct focus of the 2m RCC telescope at NAO Rozhen, Bulgaria on 2021 Oct 3.065 UT under nonphotometric weather conditions (FWHM ~ 3 arcsec). Photometric magnitudes were obtained from 3x300 sec co-added frames (see here).

AT2021zdh was discovered by Mi Zhang and Xing Gao ( XOSS) on 2021 Sept 18.608 UT at unfiltered magnitude of 18.4 and later, on 2021 Sept 19.596 UT, J. Tonry, L. Denneau, A. Heinze and co-authors reported a magnitude of 17.48 obtained in orange-ATLAS filter. Our observations show that thirteen days after the discovery the transient is very bright in H-alpha with a magnitude of 15.08±0.03 and the continuum have decreased to R=18.13±0.05 mag (3x120 sec; 2021 Oct 3.055 UT). If the whole H-alpha emission has been detected through our 30 Å filter then the pure magnitudes are H-alpha=15.12 mag and R=18.67 mag. This result confirms the object classification as a nova.

We have detected on our images also the transient AT2021ypn, which was discovered on 2021 Sept 11.602 UT by Mi Zhang and Xing Gao ( XOSS) and has been confirmed as a nova with a very strong H-alpha emission (ATel #14913) and FeII emission lines (ATel #14919); We report here that with the spectral evolution of the nova the H-alpha emission have increased to 15.42±0.03 mag, typical for some of the classical and recurrent novae. In addition we have obtained a decrease of R magnitude to 18.86±0.07 (3x120 sec; 2021 Oct 3.055 UT) which places the nova in the fast novae class (Payne-Gaposchkin 1964). If the whole H-alpha emission has been detected through our 30 Å filter then the pure magnitudes are H-alpha=15.44 mag and R=19.78 mag testifying much weaker continuum than AT2021zdh.

This work was partially supported by the Ministry of Education and Science of the Republic of Bulgaria, National RI Roadmap Project D01-383/18.12.2020 and NSF Bulgaria grant DN18/10-11.12.2017.