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

Nature Astronomy volume 2, pages 656–661 (2018)Cite this article

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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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Fig. 1: HST/ACS F775W imaging around the field of J2150–0551.
Fig. 2: Long-term luminosity and spectral evolution of J2150–0551.
Fig. 3: Disk luminosity versus temperature derived from the fits to the X-ray spectra of J2150–0551 with the standard thermal disk model.

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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.

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Authors and Affiliations

  1. Space Science Center, University of New Hampshire, Durham, NH, USA

    Dacheng Lin

  2. Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA

    Jay Strader

  3. Gemini Observatory/AURA, Southern Operations Center, La Serena, Chile

    Eleazar R. Carrasco

  4. Instituto de Astronomía, Universidad Nacional Autónoma de México, Mexico City, Mexico

    Dany Page

  5. Department of Physics and Astronomy, San José State University, San José, CA, USA

    Aaron J. Romanowsky

  6. University of California Observatories, Santa Cruz, CA, USA

    Aaron J. Romanowsky & Jean P. Brodie

  7. Eureka Scientific, Oakland, CA, USA

    Jeroen Homan

  8. SRON, Netherlands Institute for Space Research, Utrecht, The Netherlands

    Jeroen Homan

  9. Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL, USA

    Jimmy A. Irwin

  10. MIT Kavli Institute for Astrophysics and Space Research, MIT, Cambridge, MA, USA

    Ronald A. Remillard

  11. IRAP, Universit de Toulouse, CNRS, UPS, CNES, Toulouse, France

    Olivier Godet, Natalie A. Webb & Didier Barret

  12. School of Mathematics and Physics, University of Queensland, St Lucia, Queensland, Australia

    Holger Baumgardt

  13. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands

    Rudy Wijnands

  14. Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, Strasbourg, France

    Pierre-Alain Duc

  15. Canadian Astronomy Data Centre, Herzberg Institute of Astrophysics, Victoria, British Columbia, Canada

    Stephen D. J. Gwyn

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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.

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Correspondence to Dacheng Lin.

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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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