The rapidly spinning intermediate-mass black hole 3XMM J150052.0+015452

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_• and dimensionless spin parameter a_•. Previous studies have suggested that the M_• responsible for the tidal disruption event 3XMM J150052.0+015452 (hereafter J150052) is ∼10⁵ M⊙ in the intermediate-mass 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_• = 2.0^+1.0_-0.3× 10⁵ M⊙ (at 68 per cent confidence) and a_• > 0.97 (an 84.1 per cent confidence lower limit). The spectra obtained between 2008 and 2014 are significantly harder than those after 2014, an evolution that can be well explained by including the effects of inverse Comptonization 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_• and a_• values rule out both vector bosons and axions of masses ∼10⁻¹⁶ eV.