The brightest Fast Blue Optical Transients (FBOTs) are mysterious extragalactic explosions that may represent a new class of astrophysical phenomena. Their fast time to maximum brightness of less than a week and decline over several months and atypical optical spectra and evolution are difficult to explain within the context of core-collapse of massive stars which are powered by radioactive decay of Nickel-56 and evolve more slowly. AT2018cow (at redshift of 0.014) is an extreme FBOT in terms of rapid evolution and high luminosities. Here we present evidence for a high-amplitude quasi-periodic oscillation (QPO) of AT2018cow's soft X-rays with a frequency of 224 Hz (at 3.7σ significance level or false alarm probability of 0.02%) and fractional root-mean-squared amplitude of >30%. This signal is found in the average power density spectrum taken over the entire 60-day outburst and suggests a highly persistent signal that lasts for a billion cycles. The high frequency (rapid timescale) of 224 Hz (4.4 ms) argues for a compact object in AT2018cow, which can be a neutron star or black hole with a mass less than 850 solar masses. If the QPO is the spin period of a neutron star, we can set limits on the star's magnetic field strength. Our work highlights a new way of using high time-resolution X-ray observations to study FBOTs.
@article{arxiv.2112.04531,
title = {Evidence for a Compact Object in the Aftermath of the Extra-Galactic Transient AT2018cow},
author = {Dheeraj R. Pasham and Wynn C. G. Ho and William Alston and Ronald Remillard and Mason Ng and Keith Gendreau and Brian D. Metzger and Diego Altamirano and Deepto Chakrabarty and Andrew Fabian and Jon Miller and Peter Bult and Zaven Arzoumanian and James F. Steiner and Tod Strohmayer and Francesco Tombesi and Jeroen Homan and Edward M. Cackett and Alice Harding},
journal= {arXiv preprint arXiv:2112.04531},
year = {2021}
}
Comments
Published in Nature astronomy on 13th December 2021