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

Nature volume 503, pages 500–503 (2013)Cite this article

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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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Figure 1: The secondary of M 101 ULX-1 is confirmed to be a Wolf-Rayet star.
Figure 2: An orbital period of ∼8.2 days is revealed by radial velocity measurements taken over three months for M 101 ULX-1.
Figure 3: The prototype ultraluminous supersoft X-ray source M 101 ULX-1 exhibits distinct spectral characteristics.

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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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Authors and 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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  1. Ji-Feng Liu
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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.

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Correspondence to Ji-Feng Liu.

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Extended data figures and tables

Extended Data Figure 1 M 101 ULX-1 as observed in the optical region.

Left, M 101 ULX-1 is located on a spiral arm of the face-on grand-design spiral galaxy M 101, as indicated by the arrow. The colour image of M 101 is composed of GALEX NUV, SDSS g, and 2MASS J images. Right, ULX-1 is identified as a blue object with V = 23.5 mag at the centre of the 1″ circle on the HST image. The colour image is composed of ACS/WFC F435W, F555W and F814W images.

Extended Data Figure 2 Physical properties of the Wolf-Rayet secondary from spectral line modelling.

Distributions of computed Δ2 as a function of stellar masses (a), stellar mass loss rate (b), stellar radii (c) and terminal velocity (d). Here Δ2 = Σi(EW − EWi)2 computes the difference between observed and synthetic equivalent widths EW for six broad helium lines present in the Gemini/GMOS spectrum. We have computed synthetic spectra for a group of 5,000 real stars from the evolution tracks (as shown by the thick stripes in the mass plot and the radius plot) and for another group of ‘fake’ stars with continuous distributions in mass, radius and luminosity. The best model is labelled by a filled pentagon in all panels.

Source data

Extended Data Figure 3 Properties of the Wolf-Rayet/black-hole binary for different black-hole masses.

Shown are the binary separation (solid line), the Roche lobe sizes for the Wolf-Rayet star (dotted) and for the black hole (short dashed), the capture radius for the black hole when using the terminal velocity (dash–dotted) or when using a simplified velocity law v(r) = v∞(1 − R*/r) (long dashed).

Source data

Extended Data Figure 4 The black-hole accretion rate for different black-hole masses.

The accretion rates are computed adopting the terminal velocity (dotted) and a simplified velocity law v(r) = v∞(1 − R*/r) (solid). To power the observed average luminosity of 3 × 1038 erg s−1, the black-hole mass must exceed 13 (8) using the terminal velocity (the velocity law) for a Kerr black hole (η = 0.42), and exceed 46 (28) for a Schwarzschild black hole (η = 0.06). The two horizontal dotted lines indicate the accretion rates required for η = 0.06 and η = 0.42, respectively.

Source data

Extended Data Figure 5 Disk temperature structures for M 101 ULX-1.

a, The disk temperature profiles for M 101 ULX-1 (for P = 8.24 days, M* = 19, R* = 10.7, or 100) and NGC300 X-1 (for P = 32.4 h M* = 26, R* = 7.2, ; ref 22). b, The disk temperature at the outer edge for different black-hole mass in M 101 ULX-1. The horizontal line indicates the temperature required for the helium partial ionization zone.

Source data

Extended Data Table 1 Gemini/GMOS spectroscopic observations of M 101 ULX-1
Full size table
Extended Data Table 2 Properties of emission lines
Full size table

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Liu, JF., Bregman, J., Bai, Y. et al. Puzzling accretion onto a black hole in the ultraluminous X-ray source M 101 ULX-1. Nature 503, 500–503 (2013). https://doi.org/10.1038/nature12762

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