Spectroscopic classification of AT2021biy in NGC 4631 by NUTS2 (NOT Un-biased Transient Survey-2)

ATel #14360; Y.-Z. Cai (Tsinghua University), A. Pastorello (INAF-OAPd), A. Reguitti (Univ. Andres Bello), M. Fraser (UCD), S. Benetti, G. Valerin (INAF-OAPd), E. Kankare (UTU), P. Lundqvist (Stockholm University), M. D. Stritzinger (Aarhus University)
on 31 Jan 2021; 14:20 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Andrea Pastorello (andrea.pastorello@oapd.inaf.it)

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The NUTS2 collaboration (Holmbo et al. 2019, ATel #12661) reports the spectroscopic classification of AT 2021biy (ATLAS21dfy; Tonry et al. 2021, TNSAN, 33, 1) in the SBd galaxy NGC 4631. The candidate was discovered by the ATLAS survey (Tonry et al. 2018, PASP, 13, 064505; Smith et al. 2020, PASP, 132:085002).

The observation was performed with the 2.56 m Nordic Optical Telescope equipped with ALFOSC on 2020-01-31.12 UT (range 340-980 nm; resolution 1.3 nm).

 
IAU Name   | Survey name | Redshift  |  Discovery (UT) |  Discovery mag  |  Type  | Phase     | Notes 
AT 2021biy | ATLAS21dfy  | 0.002021  |  2021-01-17.56  | 18.12 (ATLAS-o) |  LRN?  | 2 weeks   | (1) 

(1) The spectrum shows a relatively blue continuum dominated by prominent Balmer lines in emission, with an unresolved narrow component superposed on a broader base (with FWHM ~2500 km/s). Narrow lines of Fe II, Ca II and O I are also identified. A narrow absorption of Na I is visible at the redshift of the host galaxy, with an equivalent width (EW) of 1.6 A, suggesting additional host galaxy extinction. Adopting the correlation between EW of Na I and color excess E(B-V) of Turatto et al. 2003, we obtain E(B-V) = 0.25 mag. The object has also been targeted in photometry on January 30.05 UT at the 62/92-cm Schmidt telescope of Asiago, at a magnitude V=18.0 (and B-V = 0.3 mag). Adopting for NGC 4631 a TRGB distance of 7.52 Mpc (Monachesi et al. 2016, MNRAS, 457, 1419), we infer for the transient an absolute magnitude M_V ~-12.1. The overall properties are consistent with those of a luminous red nova (LRN) soon after the early blue maximum (Pastorello et al. 2019, A&A, 630, A75), although an LBV-like outburst cannot be ruled out. [Ca II] lines are not clearly detected in the spectrum, hence the classification as an Intermediate-Luminosity Red Transient is less likely (Pastorello & Fraser 2019, Nature Astron. 3, 676). Due to its high luminosity (even without accounting for the host galaxy extinction), we rule out that AT 2021biy is a classical nova. The redshift of the host galaxy is from De Vaucouleurs et al. 1991, RC3, v. 3.9.