Cosmic-ray acceleration has been a long-standing mystery1,2 and, despite more than a century of study, we still do not have a complete census of acceleration mechanisms. The collision of strong stellar winds in massive binary systems creates powerful shocks that have been expected to produce high-energy cosmic rays through Fermi acceleration at the shock interface. The accelerated particles should collide with stellar photons or ambient material, producing non-thermal emission observable in X-rays and γ-rays3,4. The supermassive binary star Eta Carinae (η Car) drives the strongest colliding wind shock in the solar neighbourhood5,6. Observations with non-focusing high-energy observatories indicate a high-energy source near η Car, but have been unable to conclusively identify η Car as the source because of their relatively poor angular resolution7,8,9. Here we present direct focussing observations of the non-thermal source in the extremely hard X-ray band, which is found to be spatially coincident with the star within several arc-seconds. These observations show that the source of non-thermal X-rays varies with the orbital phase of the binary, and that the photon index of the emission is similar to that derived through analysis of the γ-ray spectrum. This is conclusive evidence that the high-energy emission indeed originates from non-thermal particles accelerated at colliding wind shocks.

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This research has made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA’s Goddard Space Flight Center. This research has made use of NASA’s Astrophysics Data System Bibliographic Services. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center. We appreciate M. Yukita, K. Madsen and M. Stuhlinger on helping resolve the NuSTAR and XMM-Newton data analysis. K.H. is supported by the Chandra grant GO4–15019A, GO7–18012A, the XMM-Newton grant NNX15AK62G, NNX16AN87G, NNX17AE67G, NNX17AE68G, and the ADAP grant NNX15AM96G. C.M.P.R. acknowledges initial support from Chandra Theory grant TM7-18003Z used in combination with an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Universities Space Research Association under contract with NASA, and current support from FONDECYT grant 3170870. A.F.J.M. is supported by NSERC (Canada) and FQRNT (Quebec).

Author information


  1. CRESST II and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD, USA

    • Kenji Hamaguchi
    •  & Michael F. Corcoran
  2. Department of Physics, University of Maryland, Baltimore County, Baltimore, MD, USA

    • Kenji Hamaguchi
    •  & Neetika Sharma
  3. The Catholic University of America, Washington, DC, USA

    • Michael F. Corcoran
  4. School of Physics and Astronomy, The University of Leeds, Leeds, UK

    • Julian M. Pittard
  5. Department of Physical Science, Hiroshima University, Hiroshima, Japan

    • Hiromitsu Takahashi
  6. Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA

    • Christopher M. P. Russell
    •  & Theodore R. Gull
  7. Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Santiago, Chile

    • Christopher M. P. Russell
  8. Space Radiation Lab, California Institute of Technology, Pasadena, CA, USA

    • Brian W. Grefenstette
  9. Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA

    • Daniel R. Wik
  10. Ritter Observatory, Department of Physics and Astronomy, The University of Toledo, Toledo, OH, USA

    • Noel D. Richardson
  11. Department of Physics and Astronomy, San Jose State University, One Washington Square, San Jose, CA, USA

    • Thomas I. Madura
  12. Département de physique and Centre de Recherche en Astrophysique du Québec (CRAQ), Université de Montréal, C.P., Montreal, Canada

    • Anthony F. J. Moffat


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K.H. and M.F.C. led the project, from proposing and planning observations, analysing the data to composing the manuscript. J.M.P. constructed a theoretical model that explains the variation of the non-thermal component. N.S. performed initial analysis of the NuSTAR data in 2015. H.T analysed and discussed Fermi data of η Car. C.M.P.R. performed theoretical simulations of η Car’s thermal X-ray emission. B.W.G. and D.R.W. discussed NuSTAR data analysis, especially the background characteristics. T.R.G. worked for the observation planning. T.R.G., N.D.R., T.I.M. and A.F.J.M. discussed the wind property of η Car. All authors reviewed the manuscript and discussed the work.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Kenji Hamaguchi.

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