Abstract

The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes1,2. Their X-ray luminosities in the 0.5–10 kiloelectronvolt energy band range from 1039 to 1041 ergs per second3. Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on black hole rather than neutron star systems1,2. The most challenging sources to explain are those at the luminous end of the range (more than 1040 ergs per second), which require black hole masses of 50–100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to an X-ray luminosity in the 3–30 kiloelectronvolt range of 4.9 × 1039 ergs per second. The pulsating source is spatially coincident with a variable source4 that can reach an X-ray luminosity in the 0.3–10 kiloelectronvolt range of 1.8 × 1040 ergs per second1. This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects.

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Acknowledgements

This work was supported by NASA (grant no. NNG08FD60C), and made use of data from the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, a project led by Caltech, managed by the Jet Propulsion Laboratory and funded by NASA. We thank the NuSTAR operations, software and calibration teams for support with execution and analysis of these observations. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. M.B. thanks the Centre National d’Études Spatiales (CNES) and the Centre National de la Recherche Scientifique (CNRS) for support. Line plots were done using Veusz software by J. Sanders.

Author information

Affiliations

  1. Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, 9, Avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

    • M. Bachetti
    • , D. Barret
    •  & N. A. Webb
  2. CNRS, Institut de Recherche en Astrophysique et Planétologie, 9, Avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France

    • M. Bachetti
    • , D. Barret
    •  & N. A. Webb
  3. Cahill Center for Astrophysics, 1216 East California Boulevard, California Institute of Technology, Pasadena, California 91125, USA

    • F. A. Harrison
    • , D. J. Walton
    • , B. W. Grefenstette
    • , F. Fürst
    • , S. R. Kulkarni
    • , V. Rana
    •  & S. P. Tendulkar
  4. MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • D. Chakrabarty
  5. Physics Department, Columbia University, 538 West 120th Street, New York, New York 10027, USA

    • A. Beloborodov
  6. Space Sciences Laboratory, University of California, Berkeley, California 94720, USA

    • S. E. Boggs
    •  & J. Tomsick
  7. DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby, Denmark

    • F. E. Christensen
  8. Lawrence Livermore National Laboratory, Livermore, California 94550, USA

    • W. W. Craig
  9. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

    • A. C. Fabian
  10. Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA

    • C. J. Hailey
  11. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

    • A. Hornschemeier
    •  & W. W. Zhang
  12. Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada

    • V. Kaspi
  13. Department of Physics, Texas Tech University, Lubbock, Texas 79409, USA

    • T. Maccarone
  14. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, Michigan 48109-1042, USA

    • J. M. Miller
  15. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    • D. Stern

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Contributions

M.B., reduction and timing analysis of the NuSTAR observations, interpretation of results, manuscript preparation; F.A.H., interpretation of results, manuscript preparation; D.J.W., NuSTAR and Chandra spectroscopy, point source analysis; B.W.G., NuSTAR image analysis; D.C., accretion torque analysis, interpretation; F.F., verification of timing analysis, interpretation; D.B., A.B., A.C.F., A.H., V.M.K., T.M., J.T., interpretation of results and manuscript review; S.B., F.C., W.W.C., C.J.H., D.S., S.P.T., N.W., W.W.Z., manuscript review.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to M. Bachetti or F. A. Harrison.

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https://doi.org/10.1038/nature13791

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