  A unique signature for the presence of massive black holes in very dense
stellar regions is occasional giant-amplitude outbursts of multiwavelength
radiation from tidal disruption and subsequent accretion of stars that make a
close approach to the black holes. Previous strong tidal disruption event (TDE)
candidates were all associated with the centers of largely isolated galaxies.
Here we report the discovery of a luminous X-ray outburst from a massive star
cluster at a projected distance of 12.5 kpc from the center of a large
lenticular galaxy. The luminosity peaked at ~10^{43} erg/s and decayed
systematically over 10 years, approximately following a trend that supports the
identification of the event as a TDE. The X-ray spectra were all very soft,
with emission confined to be <3.0 keV, and could be described with a standard
thermal disk. The disk cooled significantly as the luminosity decreased, a key
thermal-state signature often observed in accreting stellar-mass black holes.
This thermal-state signature, coupled with very high luminosities, ultrasoft
X-ray spectra and the characteristic power-law evolution of the light curve,
provides strong evidence that the source contains an intermediate-mass black
hole (IMBH) with a mass of a few ten thousand solar mass. This event
demonstrates that one of the most effective means to detect IMBHs is through
X-ray flares from TDEs in star clusters.
