A long-standing paradigm in astrophysics is that collisions—or mergers—of two neutron stars form highly relativistic and collimated outflows (jets) that power γ-ray bursts of short (less than two seconds) duration1,2,3. The observational support for this model, however, is only indirect4,5. A hitherto outstanding prediction is that gravitational-wave events from such mergers should be associated with γ-ray bursts, and that a majority of these bursts should be seen off-axis, that is, they should point away from Earth6,7. Here we report the discovery observations of the X-ray counterpart associated with the gravitational-wave event GW170817. Although the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow (known as a ‘kilonova’) from freshly synthesized rapid neutron capture (r-process) material in the merger ejecta8,9,10, observations at X-ray and, later, radio frequencies are consistent with a short γ-ray burst viewed off-axis7,11. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short γ-ray bursts and gravitational waves from neutron-star mergers, and gives independent confirmation of the collimated nature of the γ-ray-burst emission.

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We acknowledge the advice and contribution of N. Gehrels, who was co-investigator of our Chandra and Hubble Space Telescope observing programs. We thank B. Wilkes and the Chandra X-ray Center staff, N. Reid and the Space Telescope Science Institute (STScI) staff, J. Stevens and the CSIRO staff, L. Ferrarese and the Gemini support staff, in particular R. Salinas, M. Andersen, H. Kim, P. Candia and K. Silva. E. Troja thanks Bianca A. Vekstein, A. Bersich and F. Troja for help during the preparation of this manuscript. We thank V. Bajaj (STScI) and S. Hernandez for their assistance with data reduction. Work at LANL was done under the auspices of the National Nuclear Security Administration of the US Department of Energy at Los Alamos National Laboratory (LANL) under contract number DE-AC52-06NA25396. All LANL calculations were performed on LANL Institutional Computing resources. This research used resources provided by the LANL Institutional Computing Program, which is supported by the US Department of Energy National Nuclear Security Administration under contract number DE-AC52-06NA25396. M.I., S.-K.L., J.K., C.C., G.L., and Y.Y. acknowledge support from NRFK grant number 2017R1A3A3001362, funded by the Korean government. Work by C.-U.L. and S.-L.K. was supported by the KASI (Korea Astronomy and Space Science Institute) grant 2017-1-830-03. This research made use of the KMTNet system operated by KASI, and the data were obtained at three Cerro-Tololo Inter-American Observatory host sites in Chile, the South African Astronomical Observatory in South Africa, and the Siding Spring Observatory in Australia. E. Troja acknowledges support from grants GO718062A and HSTG014850001A. R.S.-R. acknowledges support by the Italian Space Agency through contract number 2015-046-R.0 and by the European Union Horizon 2020 Programme under the AHEAD project (grant agreement number 654215). T.S. acknowledges support by MEXT KAKENHI (grant numbers 17H06357 and 17H06362).

Author information


  1. Department of Astronomy, University of Maryland, College Park, Maryland 20742-4111, USA

    • E. Troja
    • , A. Kutyrev
    •  & S. Veilleux
  2. Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA

    • E. Troja
    • , S. B. Cenko
    • , A. Lien
    •  & A. Kutyrev
  3. INAF, Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Rome, Italy

    • L. Piro
    •  & R. Sánchez-Ramírez
  4. Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK

    • H. van Eerten
  5. Center for Theoretical Astrophysics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA

    • R. T. Wollaeger
    • , C. L. Fryer
    • , O. Korobkin
    •  & C. J. Fontes
  6. Center for the Exploration for the Origin of the Universe, Astronomy Program, Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea

    • M. Im
    • , S.-K. Lee
    • , C. Choi
    • , J. Kim
    • , G. Lim
    •  & Y. Yoon
  7. Space Telescope Science Institute, Baltimore, Maryland 21218, USA

    • O. D. Fox
    • , R. E. Ryan Jr
    •  & H. G. Khandrika
  8. School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA

    • N. R. Butler
  9. Joint Space-Science Institute, University of Maryland, College Park, Maryland 20742, USA

    • S. B. Cenko
    •  & S. Veilleux
  10. Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara-shi Kanagawa 252-5258, Japan

    • T. Sakamoto
  11. INAF-Istituto di Radioastronomia, Via Gobetti 101, I-40129, Italy

    • R. Ricci
  12. Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA

    • A. Lien
  13. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany

    • J. M. Burgess
  14. Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 Ciudad de México, México

    • W. H. Lee
    •  & A. M. Watson
  15. INAF/Brera Astronomical Observatory, via Bianchi 46, Merate, Italy

    • S. Covino
    •  & P. D’Avanzo
  16. Instituto de Astrofísica de Canarias, E-38200 La Laguna, Spain

    • J. Becerra González
  17. Universidad de La Laguna, Departimento of Astrofísica, E-38206 La Laguna, Spain

    • J. Becerra González
  18. Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, South Korea

    • S.-L. Kim
    •  & C.-U. Lee
  19. Astronomy Program, Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Korea

    • H. M. Lee
  20. CSIRO Astronomy and Space Science, PO Box 76, Epping, New South Wales 1710, Australia

    • M. H. Wieringa


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E. Troja, L.P., H.v.E. and O.K. composed the text, with input from all co-authors. E. Troja and T.S. obtained and analysed the Chandra X-ray observations. Hubble Space Telescope observations were obtained, reduced and analysed by E. Troja, O.D.F., R.E.R. Jr and H.G.K. E. Troja, N.R.B., S.B.C., J.B.G. and R.S.-R. obtained, processed and analysed the Gemini data. M.I., C.-U.L., S.-L.K., J.K., C.C., G. L., H.M.L. led the optical imaging with KMTNet. E. Troja, L.P., R.R. and M.H.W. obtained, processed and analysed the Australia Telescope Compact Array observations. R.T.W., O.K., C.L.F. and C.J.F. led the modelling of the kilonova emission. H.v.E., L.P. and E. Troja led the modelling of the GRB and afterglow emission. A.M.W., W.H.L. and J.M.B. contributed to the spectral modelling. M.I., Y.Y. and S.-K.L. led the analysis of the host galaxy. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to E. Troja.

Reviewer Information Nature thanks R. Chevalier and C. Miller for their contribution to the peer review of this work.

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