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

## Abstract

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 galaxies2,3,4,5,6. 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 intermediate-mass 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.

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## Acknowledgements

D.L. is supported by the National Aeronautics and Space Administration (NASA) through Chandra Award number GO6-17046X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the NASA under contract NAS8-03060, and by the NASA ADAP grant NNX17AJ57G. A.J.R. was supported by National Science Foundation (NSF) grant AST-1515084, and as a Research Corporation for Science Advancement Cottrell Scholar. J.S. acknowledges support from NSF grant AST-1514763 and a Packard Fellowship. D.P. was partially supported by the Consejo Nacional de Ciencia y Tecnología with CB-2014-1 grant number 240512. N.A.W., O.G. and D.B. acknowledge CNES for financial support to the XMM-Newton Survey Science Center activities. R.W. acknowledges support from the Netherlands Organisation for Scientific Research through a TOP Grant, module 1. J.P.B. acknowledges support from NSF grant AST 1518294. We thank the former Swift principal investigator N. Gehrels for approving our target of opportunity request to observe J2150–0551. We thank Z. Jennings for assistance with the Suprime-Cam data. The findings in this paper are based on observations obtained from XMM-Newton, Chandra, Swift, HST, CFHT, Gemini, SOAR and Subaru.

## Author information

Authors

### Contributions

D.L. wrote the main manuscript and led the data analysis. E.R.C. helped reduce the GMOS spectra and pre-imaging. D.P. performed the MCMC simulations for NSCool. J.S. obtained the SOAR U-band image and fitted the HST image with ISHAPE. A.J.R. obtained the Subaru g′-band image. S.D.J.G. stacked the CFHT images. All authors discussed the results and commented on the manuscript.

### Corresponding author

Correspondence to Dacheng Lin.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary text, Supplementary Figures 1–12, Supplementary Tables 1–3, Supplementary references

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Lin, D., Strader, J., Carrasco, E.R. et al. A luminous X-ray outburst from an intermediate-mass black hole in an off-centre star cluster. Nat Astron 2, 656–661 (2018). https://doi.org/10.1038/s41550-018-0493-1

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• DOI: https://doi.org/10.1038/s41550-018-0493-1

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