A Possible 250 s X-Ray Quasi-periodicity in the Fast Blue Optical Transient AT2018cow

The fast blue optical transients(FBOTs)are a new population of extragalactic transients of unclear physical origin.A variety of mechanisms has been proposed including failed supernova explosion,shock interaction with a dense medium,young magnetar,accretion onto a compact object and stellar tidal disruption event,but none is conclusive.Here we report the discovery of a possible X-ray quasi-periodicity signal with a period of～250 s(at a significance level of 99.76％)in the brightest FBOT AT2018cow through the analysis of XMM-Newton/PN data.The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope,with observations covering from 6 to 37 days after the optical discovery,though the significance level is lower(94.26％).This suggests that the quasi-periodic oscillation(QPO)frequency may be stable over at least 1.1×104 cycles.Assuming the～250 s QPO to be a scaled-down analog of that typically seen in stellar mass black holes,a black hole mass of～103-105 solar masses could be inferred.The overall X-ray luminosity evolution could be modeled with a stellar tidal disruption by a black hole of～104 solar masses,providing a viable mechanism to produce AT2018cow.Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.