Observations of the luminous red nova AT 2021biy in the nearby galaxy NGC 4631

• Published in: arXiv.org 2022-08
• Main Authors: Y -Z Cai, Pastorello, A, Fraser, M, X -F Wang, Filippenko, A V, Reguitti, A, Patra, K C, Goranskij, V P, Barsukova, E A, Brink, T G, Elias-Rosa, N, Stevance, H F, Zheng, W, Yang, Y, Atapin, K E, Benetti, S, T J L de Boer, Bose, S, Burke, J, Byrne, R, Cappellaro, E, Chambers, K C, W -L Chen, Emami, N, Gao, H, Hiramatsu, D, Howell, D A, Huber, M E, Kankare, E, Kelly, P L, Kotak, R, Kravtsov, T, V Yu Lander, Z -T Li, C -C Lin, Lundqvist, P, Magnier, E A, Malygin, E A, Maslennikova, N A, Matilainen, K, Mazzali, P A, McCully, C, J Mo, Moran, S, Newsome, M, Oparin, D V, E Padilla Gonzalez, Reynolds, T M, Shatsky, N I, Smartt, S J, Smith, K W, Stritzinger, M D, Tatarnikov, A M, Terreran, G, Uklein, R I, Valerin, G, Vallely, P J, Vozyakova, O V, Wainscoat, R, S -Y Yan, J -J Zhang, T -M Zhang, Zheltoukhov, S G, Dastidar, R, Fulton, M, Galbany, L, Gangopadhyay, A, H -W Ge, Gutiérrez, C P, Lin, H, Misra, K, Z -W Ou, Salmaso, I, Tartaglia, L, Xiao, L, X -H Zhang
• Format: Article
• Language: English
• Subjects: Hubble Space Telescope; Inspection; K stars; Light curve; Luminosity; M stars; Metallicity; Milky Way; Space telescopes; Spectra
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2207.00734

We present an observational study of the luminous red nova (LRN) AT\,2021biy in the nearby galaxy NGC\,4631. The field of the object was routinely imaged during the pre-eruptive stage by synoptic surveys, but the transient was detected only at a few epochs from \(\sim 231\)\,days before maximum brightness. The LRN outburst was monitored with unprecedented cadence both photometrically and spectroscopically. AT\,2021biy shows a short-duration blue peak, with a bolometric luminosity of \(\sim 1.6 \times 10^{41}\)\,erg\,s\(^{-1}\), followed by the longest plateau among LRNe to date, with a duration of 210\,days. A late-time hump in the light curve was also observed, possibly produced by a shell-shell collision. AT\,2021biy exhibits the typical spectral evolution of LRNe. Early-time spectra are characterised by a blue continuum and prominent H emission lines. Then, the continuum becomes redder, resembling that of a K-type star with a forest of metal absorption lines during the plateau phase. Finally, late-time spectra show a very red continuum (\(T_{\mathrm{BB}} \approx 2050\) K) with molecular features (e.g., TiO) resembling those of M-type stars. Spectropolarimetric analysis indicates that AT\,2021biy has local dust properties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection of archival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a \(\sim 20\)\,\msun\ progenitor candidate with log\,\((L/{\rm L}_{\odot}) = 5.0\)\,dex and \(T_{\rm{eff}} = 5900\)\,K at solar metallicity. The above luminosity and colour match those of a luminous yellow supergiant. Most likely, this source is a close binary, with a 17--24\,\msun\ primary component.