A possible 250-second X-ray quasi-periodicity in the fast blue optical transient AT2018cow

Abstract

The fast blue optical transients (FBOTs) are a new population of extragalactic transients of unclear physical origin. A variety of mechanisms have 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 $\sim$250 second (at a significance level of 99.76%) in the brightest FBOT AT2018cow through the analysis of XMM-Newton/PN data. The signal is independently detected at the same frequency in the average power density spectrum from data taken from the Swift telescope, with observations covering from 6 to 37 days after the optical discovery, though the significance level is lower (94.26%). This suggests that the QPO frequency may be stable over at least 1.1$\times$ 10${}^4$ cycles. Assuming the $\sim$250 second QPO to be a scaled-down analogue of that typically seen in stellar mass black holes, a black hole mass of $\sim 10^3-10^5$ solar masses could be inferred. The overall X-ray luminosity evolution could be modeled with the stellar tidal disruption by a black hole of $\sim 10^4$ solar masses, providing a viable mechanism to produce AT2018cow. Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.

18 pages, 10 figures. Accepted for publication in Research in Astronomy and Astrophysics

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• Science objectives of the Einstein Probe mission
• Minutes-duration Optical Flares with Supernova Luminosities
• Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV bright Underlying Source 2--4 Yr Post-discovery
• Multi-wavelength observations of the Luminous Fast Blue Optical Transient AT2023fhn
• An 85-s X-ray quasi-periodicity after a stellar tidal disruption by a candidate intermediate-mass black hole
• Luminous Fast Blue Optical Transients as "Failed" Gravitational-wave Sources: Helium Core$-$Black Hole Mergers Following Delayed Dynamical Instability
• The spectra of a radiative reprocessing outflow model for fast blue optical transients
• AT2019ijn: a fast-rising, slow-decaying blue optical transient with exceptionally bright radio emission