Evidence for a Compact Object in the Aftermath of the Extra-Galactic Transient AT2018cow

Abstract

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 3.7$\sigma$ 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.

Published in Nature astronomy on 13th December 2021

References in corpus (10)

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Cited by in corpus (8)

• Science objectives of the Einstein Probe mission
• The Birth of a Relativistic Jet Following the Disruption of a Star by a Cosmological Black Hole
• From X-rays to physical parameters: a comprehensive analysis of thermal tidal disruption event X-ray spectra
• A Case for a Binary Black Hole System Revealed via Quasi-Periodic Outflows
• Minutes-duration Optical Flares with Supernova Luminosities
• A flash of polarized optical light points to an aspherical "cow"
• The Black Hole Candidate Swift J1728.9$-$3613 and the Supernova Remnant G351.9$-$0.9
• AT2019ijn: a fast-rising, slow-decaying blue optical transient with exceptionally bright radio emission