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

Owen R. McBrien, Stephen J. Smartt, Ting-Wan Chen, Cosimo Inserra, James H. Gillanders, Stuart A. Sim, Anders Jerkstrand, Armin Rest, Stefano Valenti, Rupak Roy, Mariusz Gromadzki, Stefan Taubenberger, Andreas Flörs, Mark E. Huber, Ken C. Chambers, Avishay Gal-Yam, David R. Young, Matt Nicholl, Erkki Kankare, Ken W. Smith, Kate Maguire, Ilya Mandel, Simon Prentice, Ósmar Rodríguez, Jonathon Pineda Garcia, Claudia P. Gutiérrez, Lluís Galbany, Cristina Barbarino, Peter S. J. Clark, Jesper Sollerman, Shrinivas R. Kulkarni, Kishalay De, David A. H. Buckley, Arne Rau

Published 2019-09-10Version 1

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 d$^{\textrm{-1}}$ 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.