SN2018kzr: a rapidly declining transient from the destruction of a white dwarf

TL;DR

SN2018kzr is the fastest declining supernova-like transient, likely powered by a magnetar or white dwarf-neutron star merger, with unique spectral features and a very low ejecta mass, challenging traditional supernova models.

Contribution

This paper presents the discovery and analysis of SN2018kzr, a rapidly declining transient, proposing new explosion scenarios involving magnetar spin-down and white dwarf-neutron star mergers.

Findings

Rapid decline rate of 0.48 mag/day in r band

Ejecta mass estimated at 0.10 solar masses

Spectral features indicate intermediate mass elements, disfavoring neutron star merger

Abstract

We present SN2018kzr, the fastest declining supernova-like transient, second only to the kilonova, AT2017gfo. SN2018kzr is characterized by a peak magnitude of $M_r = -17.98$, peak bolometric luminosity of ${\sim} 1.4 \times 10^{43}$erg s$^{\mathrm{-1}}$ and a rapid decline rate of $0.48 \pm 0.03$ mag day$^{\textrm{-1}}$ in the $r$ band. The bolometric luminosity evolves too quickly to be explained by pure $^{\mathrm{56}}$Ni heating, necessitating the inclusion of an alternative powering source. Incorporating the spin-down of a magnetized neutron star adequately describes the lightcurve and we estimate a small ejecta mass of $M_\mathrm{ej} = 0.10 \pm 0.05$ $\textrm{M}_{\odot}$. Our spectral modelling suggests the ejecta is composed of intermediate mass elements including O, Si and Mg and trace amounts of Fe-peak elements, which disfavours a binary neutron star merger. We discuss three explosion scenarios for SN2018kzr, given the low ejecta mass, intermediate mass element composition and the high likelihood of additional powering - core collapse of an ultra-stripped progenitor, the accretion induced collapse of a white dwarf and the merger of a white dwarf and neutron star. The requirement for an alternative input energy source favours either the accretion induced collapse with magnetar powering or a white dwarf - neutron star merger with energy from disk wind shocks.