Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black Hole Candidate 3XMM J215022.4−055108

Abstract

We recently discovered the X-ray/optical outbursting source 3XMM J215022.4−055108. It was best explained as the tidal disruption of a star by an intermediate-mass black hole of mass of a few tens of thousand solar masses in a massive star cluster at the outskirts of a large barred lenticular galaxy at D L  = 247 Mpc. However, we could not completely rule out a Galactic cooling neutron star as an alternative explanation for the source. In order to further pin down the nature of the source, we have obtained new multiwavelength observations by XMM-Newton and the Hubble Space Telescope ( HST ). The optical counterpart to the source in the new HST image is marginally resolved, which rules out the Galactic cooling neutron star explanation for the source and suggests a star cluster of half-light radius ∼27 pc. The new XMM-Newton observation indicates that the luminosity was decaying as expected for a tidal disruption event and that the disk was still in the thermal state with a supersoft X-ray spectrum. Therefore, the new observations confirm the source as one of the best intermediate-mass black hole candidates.