Letter | Published:

Puzzling accretion onto a black hole in the ultraluminous X-ray source M 101 ULX-1

Nature volume 503, pages 500503 (28 November 2013) | Download Citation

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Abstract

There are two proposed explanations for ultraluminous X-ray sources1,2 (ULXs) with luminosities in excess of 1039 erg s−1. They could be intermediate-mass black holes (more than 100–1,000 solar masses, ) radiating at sub-maximal (sub-Eddington) rates, as in Galactic black-hole X-ray binaries but with larger, cooler accretion disks3,4,5. Alternatively, they could be stellar-mass black holes radiating at Eddington or super-Eddington rates2,6. On its discovery, M 101 ULX-14,7 had a luminosity of 3 × 1039 erg s−1 and a supersoft thermal disk spectrum with an exceptionally low temperature—uncomplicated by photons energized by a corona of hot electrons—more consistent with the expected appearance of an accreting intermediate-mass black hole3,4. Here we report optical spectroscopic monitoring of M 101 ULX-1. We confirm the previous suggestion8 that the system contains a Wolf-Rayet star, and reveal that the orbital period is 8.2 days. The black hole has a minimum mass of 5, and more probably a mass of 20−30, but we argue that it is very unlikely to be an intermediate-mass black hole. Therefore, its exceptionally soft spectra at high Eddington ratios violate the expectations for accretion onto stellar-mass black holes9,10,11. Accretion must occur from captured stellar wind, which has hitherto been thought to be so inefficient that it could not power an ultraluminous source12,13.

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Acknowledgements

We thank J. McClintock, R. Di Stefano, Q.-Z. Liu, X.-D. Li, F. Yuan and S.-N. Zhang for discussions. J.-F.L. acknowledges support for this work provided by NASA through the Chandra Fellowship Program (grant PF6-70043), support from the Chinese Academy of Sciences through grant KJCX2-EW-T01 and support by the National Science Foundation of China through grants NSFC-11273028 and NSFC-11333004. The paper is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).

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Affiliations

  1. Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, 100012 Beijing, China

    • Ji-Feng Liu
    • , Yu Bai
    •  & Stephen Justham
  2. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, Michigan 40185, USA

    • Joel N. Bregman
  3. Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK

    • Paul Crowther

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Contributions

J.-F.L. and J.N.B. proposed the observations, J.-F.L. and Y.B. reduced the data and carried out the analysis, J.-F.L., J.N.B. and S.J. discussed the results and wrote the paper, and P.C. helped to confirm the properties of the Wolf-Rayet star. All authors commented on the manuscript and contributed to the revision of the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Ji-Feng Liu.

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https://doi.org/10.1038/nature12762

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