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

We present a photometric and spectroscopic analysis of the ultraluminous and slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign starts from similar to 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 similar to 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 fluxexcess for the first similar to 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 [OI] 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 similar to 0 . 04 M-? of H/He-poor circumstellar material at a distance of similar to 10(12) cm, while the low ionization state of the late-time spectrum re veals lo w 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.

Automated summary

We analyze the ultraluminous and slowly evolving Type Ia supernova 2021zny, known as an 03fg-like event, through photometric and spectroscopic observations. Our campaign, spanning from 5.3 hours after explosion to 10 months from peak brightness, reveals several characteristics of the SN, including peak brightness, slow decline, blue early-time colors, low ejecta velocities, and presence of unburned material. Notably, we observe an excess in flux in the early phase and prominent [OI] lines in the late-time spectrum, suggesting an interaction between the ejecta and circumstellar material, as well as low abundances of stable iron-peak elements.