A luminous X-ray outburst from an intermediate-mass black hole in an off-centre star cluster

TL;DR

This paper reports the discovery of a luminous, decade-long X-ray outburst from a star cluster, providing strong evidence for an intermediate-mass black hole through thermal disk signatures typical of tidal disruption events.

Contribution

It presents the first evidence of an intermediate-mass black hole in a star cluster outside galaxy centers via a detailed analysis of a unique X-ray outburst.

Findings

Luminosity peaked at ~10^{43} erg/s and decayed over 10 years.

X-ray spectra were very soft, <3.0 keV, consistent with a thermal disk.

The event's characteristics strongly suggest a tidal disruption event by an IMBH.

Abstract

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.