A unique signature for the presence of massive black holes in
very dense stellar regions is occasional giant-amplitude outbursts
of multi-wavelength radiation from tidal disruption and
subsequent accretion of stars that make a close approach to
the black holes1
. Previous strong tidal disruption event (TDE)
candidates were all associated with the centres of largely
isolated galaxies2–6. 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 centre of a large lenticular galaxy.
The luminosity peaked at ~1043 erg s−1
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 intermediatemass
black hole with a mass tens of thousand times that of
the solar mass. This event demonstrates that one of the most
effective means of detecting intermediate-mass black holes is
through X-ray flares from TDEs in star clusters.