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

TL;DR

This paper reports the detection of a 250-second X-ray quasi-periodicity in the FBOT AT2018cow, suggesting the presence of an intermediate-mass black hole and providing insights into the transient's possible origin.

Contribution

It presents the first evidence of a stable X-ray QPO in AT2018cow, linking it to a potential intermediate-mass black hole and proposing a tidal disruption event as the mechanism.

Findings

Detected a 250-second X-ray QPO with high significance.

QPO frequency appears stable over at least 11,000 cycles.

Black hole mass inferred to be between 1,000 and 100,000 solar masses.

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 $\sim10^{3}-10^{5}$ solar masses could be inferred. The overall X-ray luminosity evolution could be modeled with the stellar tidal disruption by a black hole of $\sim10^4$ solar masses, providing a viable mechanism to produce AT2018cow. Our findings suggest that other bright FBOTs may also harbor intermediate-mass black holes.