The Rapidly Spinning Intermediate-Mass Black Hole 3XMM J150052.0+015452

Abstract

A star tidally disrupted by a black hole can form an accretion disc with a super-Eddington mass accretion rate; the X-ray emission produced by the inner disc provides constraints on the black hole mass $M_\bullet$ and dimensionless spin parameter $a_\bullet$. Previous studies have suggested that the $M_\bullet$ responsible for the tidal disruption event 3XMM J150052.0+015452 (hereafter J150052) is $\sim$10$^{5} M_{\odot}$, in the intermediate black hole (IMBH) regime. Fitting multi-epoch XMM-Newton and Chandra X-ray spectra obtained after 2008 during the source's decade-long decay, with our latest slim accretion disc model gives $M_\bullet = 2.0^{+1.0}_{-0.3}\times10^{5} M_{\odot}$ (at 68% confidence) and $a_\bullet > 0.97$ (a 84.1% confidence lower limit). The spectra obtained between 2008-2014 are significantly harder than those after 2014, an evolution that can be well explained by including the effects of inverse-Comptonisation by a corona on the early-time spectra. The corona is present when the source accretion rate is super-Eddington, while there is no evidence for its effect in data obtained after 2014, when the mass accretion rate is around the Eddington-limit. Based on our spectral study, we infer that the corona is optically thick and warm ($kT_e=2.3^{+2.7}_{-0.8}$ keV). Our mass and spin measurements of J150052 confirm it as an IMBH and point to a rapid, near extremal, spin. These $M_\bullet$ and $a_\bullet$ values rule out both vector bosons and axions of masses $\sim10^{-16}$ eV.