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 ^ { 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 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 .