Slim disk modeling reveals an accreting intermediate-mass black hole in the luminous fast blue optical transient AT2018cow

TL;DR

This study uses X-ray spectral analysis and slim disk modeling to identify an accreting intermediate-mass black hole as the central engine in the luminous fast blue optical transient AT2018cow, providing new insights into its nature.

Contribution

The paper introduces a slim disk model application to constrain the black hole mass in AT2018cow, supporting the intermediate-mass black hole hypothesis.

Findings

Black hole mass estimated at ~250 Msun

Consistent with both tidal disruption and merger scenarios

Evidence for an accreting intermediate-mass black hole

Abstract

The origin of the most luminous subclass of the fast blue optical transients (LFBOTs) is still unknown. We present an X-ray spectral analysis of AT2018cow-the LFBOT archetype-using NuSTAR, Swift, and XMM-Newton data. The source spectrum can be explained by the presence of a slim accretion disk, and we find that the mass accretion rate decreases to sub-Eddington levels >~ 200 days after the source's discovery. Applying our slim disk model to data obtained at multiple observational epochs, we constrain the mass of the central compact object in AT2018cow to be log(M_BH/Msun)=2.4+0.6/-0.1 at the 68% confidence level. Our mass measurement is independent from, but consistent with, the results from previously employed methods. The mass constraint is consistent with both the tidal disruption and the black hole-star merger scenarios, if the latter model can be extrapolated to the measured black hole mass. Our work provides evidence for an accreting intermediate-mass black hole (10^2--10^6 Msun) as the central engine in AT2018cow, and, by extension, in LFBOT sources similar to AT2018cow.