Mon. Not. R. Astron. Soc., 497, 246-262 (2020/September-1)
AT2018kzr: the merger of an oxygen-neon white dwarf and a neutron star or black hole.
GILLANDERS J.H., SIM S.A. and SMARTT S.J.
Abstract (from CDS):
We present detailed spectroscopic analysis of the extraordinarily fast-evolving transient AT2018kzr. The transient's observed light curve 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 ∼12 000-14 500 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 per cent). This implies a high Fe/(Ni+Co) ratio. Given the short time from the transient's proposed explosion epoch, the Fe cannot be 56Fe resulting from the decay of radioactive 56Ni synthesized in the explosion. Instead, we propose that this is stable 54Fe, and that the transient is unusually rich in this isotope. We further identify an additional, high-velocity component of ejecta material at ∼20 000-26 000 km s–1, which is mildly asymmetric and detectable through the Ca II near-infrared 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.
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
line: identification - radiative transfer - binaries: close - stars: neutron - supernovae: individual: AT2018kzr - white dwarfs
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