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 10 days and decline over less than 2 months and unusual 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 slowly on months timescales. AT2018cow (at a redshift of 0.014) is an extreme FBOT both in terms of rapid evolution and high X-ray and bolometric luminosities. Several alternative hypotheses have been proposed to explain its unusual properties. These include shock interactions with dense circumstellar medium, tidal disruption of a star by a 10,000−million solar mass black hole, failed supernova with fallback accretion onto a newborn black hole, neutron star formed in a supernova or from merging compact objects, etc. Here, we present evidence for a high-amplitude (fractional root-mean-squared amplitude of>30%) quasi-periodic oscillation (QPO) of AT2018cow’s soft X-rays with a centroid frequency of roughly 225 Hz (statistically significant at the 3.7-sigma level, or a false alarm probability of 0.02%). This signal is found in the average power density spectrum of data taken over the entire outburst lasting roughly 60 days and thus suggests that the signal is highly persistent over several hundreds of millions of cycles (60 daysx225 Hz). This high frequency (rapid timescale) of 225 Hz (4.4 ms) argues for the presence of a compact object in AT2018cow which can either be a neutron star or a black hole, and disfavors circumstellar medium interactions for the origin of X-ray emission. Also, the QPO’s timescale sets an upper limit on the compact object's mass to be 850 solar masses, and thus disfavors models with a heavier black hole. If the QPO represents the spin period of a neutron star we can set upper limits on its magnetic field under different scenarios. This work highlights a new way of using high time-resolution X-ray observations to study FBOTs.
