  We simultaneously and successfully fit the multi-epoch X-ray spectra of the
tidal disruption event (TDE) 3XMM J215022.4-055108 using a modified version of
our relativistic slim disk model that now accounts for angular momentum losses
from radiation. We explore the effects of different disk properties and of
uncertainties in the spectral hardening factor fc and redshift z on the
estimation of the black hole mass M and spin a. Across all choices of
theoretical priors, we constrain M to less than 2.2e4 Ms at 1 sigma confidence.
Assuming that the TDE host is a star cluster associated with the adjacent,
giant, barred lenticular galaxy at z=0.055, we constrain M and a to be
(1.75+0.45-0.05)e4 Ms and 0.8+0.12-0.02, respectively, at 1 sigma confidence.
The high, but sub-extremal, spin suggests that, if this intermediate mass black
hole (IMBH) has grown significantly since formation, it has acquired its last
e-fold in mass in a way incompatible with both the standard and chaotic limits
of gas accretion. Ours is the first clear IMBH with a spin measurement. As
such, this object represents a novel laboratory for astro-particle physics; its
M and a place tight limits on the existence of ultralight bosons, ruling out
those with masses 1.0e-15 to 1.0e-16 eV.
