SN 2021zny: an early flux excess combined with late-time oxygen emission suggests a double white dwarf merger event

• Published in: Monthly notices of the Royal Astronomical Society 2023-05, Vol.521 (1), p.1162-1183
• Main Authors: Dimitriadis, Georgios, Maguire, Kate, Karambelkar, Viraj R, Lebron, Ryan J, Liu (刘 畅), Chang, Kozyreva, Alexandra, Miller, Adam A, Ridden-Harper, Ryan, Anderson, Joseph P, Chen, Ting-Wan, Coughlin, Michael, Valle, Massimo Della, Drake, Andrew, Galbany, Lluís, Gromadzki, Mariusz, Groom, Steven L, Gutiérrez, Claudia P, Ihanec, Nada, Inserra, Cosimo, Johansson, Joel, Müller-Bravo, Tomás E, Nicholl, Matt, Polin, Abigail, Rusholme, Ben, Schulze, Steve, Sollerman, Jesper, Srivastav, Shubham, Taggart, Kirsty, Wang, Qinan, Yang (杨 轶), Yi, Young, David R
• Format: Article
• Language: English
• Subjects: 2021zny; individual; supernovae - supernovae; transients
• ISSN: 0035-8711; 1365-2966; 1365-2966
• DOI: 10.1093/mnras/stad536

ABSTRACT We present a photometric and spectroscopic analysis of the ultraluminous and slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign starts from ∼5.3 h after explosion (making SN 2021zny one of the earliest observed members of its class), with dense multiwavelength coverage from a variety of ground- and space-based telescopes, and is concluded with a nebular spectrum ∼10 months after peak brightness. SN 2021zny displayed several characteristics of its class, such as the peak brightness (MB = −19.95 mag), the slow decline (Δm15(B) = 0.62 mag), the blue early-time colours, the low ejecta velocities, and the presence of significant unburned material above the photosphere. However, a flux excess for the first ∼1.5 d after explosion is observed in four photometric bands, making SN 2021zny the third 03fg-like event with this distinct behaviour, while its +313 d spectrum shows prominent [O i] lines, a very unusual characteristic of thermonuclear SNe. The early flux excess can be explained as the outcome of the interaction of the ejecta with $\sim 0.04\, \mathrm{M_{\odot }}$ of H/He-poor circumstellar material at a distance of ∼1012 cm, while the low ionization state of the late-time spectrum reveals low abundances of stable iron-peak elements. All our observations are in accordance with a progenitor system of two carbon/oxygen white dwarfs that undergo a merger event, with the disrupted white dwarf ejecting carbon-rich circumstellar material prior to the primary white dwarf detonation.