AT2018kzr: the merger of an oxygen–neon white dwarf and a neutron star or black hole. Issue 1 (24th July 2020)

• Record Type: Journal Article
• Authors: Gillanders, J H; Sim, S A; Smartt, S J
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 497:Issue 1(2020)
• Page Start: 246
• Page End: 262
• Publication Date: 2020-07-24
• Subjects: line: identification -- radiative transfer -- binaries: close -- stars: neutron -- supernovae: individual: AT2018kzr -- white dwarfs; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa1822
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: 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 $\rm {km}\, s^{-1}$ . 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 56 Fe resulting from the decay of radioactive 56 Ni synthesized 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 ∼20 000–26 000 $\rm {km}\, s^{-1}$, 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.