Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black
  Hole Candidate 3XMM J215022.4-055108
  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 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
super-soft X-ray spectrum. Therefore, the new observations confirm the source
as one of the best intermediate-mass black hole candidates.
