We present detailed spectroscopic analysis of the extraordinarily fast-evolving transient AT2018kzr .
The transient ’ s observed lightcurve showed a rapid decline rate , comparable to the kilonova AT2017gfo .
We calculate a self-consistent sequence of radiative transfer models ( using tardis ) and determine that the ejecta material is dominated by intermediate-mass elements ( O , Mg and Si ) , with a photospheric velocity of \sim 12000–14500 km s ^ { -1 } .
The early spectra have the unusual combination of being blue but dominated by strong Fe ii and Fe iii absorption features .
We show that this combination is only possible with a high Fe content ( 3.5 % ) .
This implies a high Fe/ ( Ni+Co ) ratio .
Given the short time from the transient ’ s proposed explosion epoch , the Fe can not be ^ { 56 } Fe resulting from the decay of radioactive ^ { 56 } Ni synthesised in the explosion .
Instead , we propose that this is stable ^ { 54 } Fe , and that the transient is unusually rich in this isotope .
We further identify an additional , high-velocity component of ejecta material at \sim 20000–26000 km s ^ { -1 } , which is mildly asymmetric and detectable through the Ca ii NIR triplet .
We discuss our findings with reference to a range of plausible progenitor systems and compare with published theoretical work .
We conclude that AT2018kzr is most likely the result of a merger between an ONe white dwarf and a neutron star or black hole .
As such , it would be the second plausible candidate with a good spectral sequence for the electromagnetic counterpart of a compact binary merger , after AT2017gfo .