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

Abstract

We present a photometric and spectroscopic analysis of the ultra-luminous and slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign starts from $\sim 5.3$ hours after explosion (making SN 2021zny one of the earliest observed members of its class), with dense multi-wavelength coverage from a variety of ground- and space-based telescopes, and is concluded with a nebular spectrum $\sim 10$ 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 ($\Delta m_{15}(B)=0.62$ 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 $\sim 1.5$ days after explosion is observed in four photometric bands, making SN 2021zny the third 03fg-like event with this distinct behavior, 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 $\sim 10^{12}$ 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.

19 pages, 16 figures, accepted for publication in MNRAS

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