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

Dimitriadis, Georgios and Maguire, Kate and Karambelkar, Viraj R and Lebron, Ryan J and 畅), Chang Liu (刘 and Kozyreva, Alexandra and Miller, Adam A and Ridden-Harper, Ryan and Anderson, Joseph P and Chen, Ting-Wan and Coughlin, Michael and Valle, Massimo Della and Drake, Andrew and Galbany, Lluís and Gromadzki, Mariusz and Groom, Steven L and Gutiérrez, Claudia P and Ihanec, Nada and Inserra, Cosimo and Johansson, Joel and Müller-Bravo, Tomás E and Nicholl, Matt and Polin, Abigail and Rusholme, Ben and Schulze, Steve and Sollerman, Jesper and Srivastav, Shubham and Taggart, Kirsty and Wang, Qinan and 轶), Yi Yang (杨 and Young, David R (2023) SN 2021zny: an early flux excess combined with late-time oxygen emission suggests a double white dwarf merger event. Monthly Notices of the Royal Astronomical Society, 521 (1). pp. 1162-1183. ISSN 0035-8711

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 (M B = −19.95 mag), the slow decline (Δm 15(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 ∼ 0.04 M ☉ of H/He-poor circumstellar material at a distance of ∼10 12 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.