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

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${}^{-1}$ and a rapid decline rate of $0.48\pm 0.03$ mag day${}^{\text{-1}}$ in the $r$ band. The bolometric luminosity evolves too quickly to be explained by pure ${}^{56}$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 $M_{\mathrm{ej}}=0.10\pm 0.05$ ${\text{M}}_{\odot }$. 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.

11 pages, 5 figures, 3 tables Accepted by ApJL on 2019 October 15