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

¹ SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
³ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, China
⁴ University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA

^⋆ Corresponding author; z.cao@sron.nl

Received: 28 June 2024
Accepted: 24 September 2024

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_•/M_⊙) = 2.4_−0.1^+0.6 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 (10² − 10⁶ M_⊙) as the central engine in AT2018cow, and, by extension, in LFBOT sources similar to AT2018cow.