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

TL;DR

This paper presents evidence of a high-frequency quasi-periodic oscillation in the X-ray emission of the transient AT2018cow, suggesting the presence of a compact object such as a neutron star or black hole.

Contribution

The study provides the first detection of a QPO in an FBOT, indicating a compact object and offering new insights into the nature of these explosive events.

Findings

Detection of a 224 Hz QPO in AT2018cow's X-ray emission

QPO persists over the entire 60-day outburst

Implying a compact object with mass less than 850 solar masses

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.