The ejecta composition is an open question in gamma-ray burst (GRB) physics1. Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis2, suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the Band function3,4,5, consistent with a synchrotron radiation origin5,6 and suggesting that the jet is Poynting-flux dominated at the central engine and probably in the emission region as well7,8. There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component9, suggesting a probable hybrid jet composition10. Here, we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-ray and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each sub-burst being approximately 0.8 s, 35 s and 212 s, respectively. Its high brightness (with isotropic peak luminosity Lp,iso ≈ 4 × 1053 erg s−1) allows us to conduct detailed time-resolved spectral analysis in each episode, from precursor to main burst and to extended emission. The spectral properties of the first two sub-bursts are distinctly different, allowing us to observe the transition from thermal to non-thermal radiation between well-separated emission episodes within a single GRB. Such a transition is a clear indication of the change of jet composition from a fireball to a Poynting-flux-dominated jet.

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A correction to this article is available online at https://doi.org/10.1038/s41550-018-0387-2.

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  • 31 January 2018

    In the version of this Letter originally published, the letter í was mistakenly omitted from the surname of the author R. Sánchez-Ramírez. This has now been corrected.


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B.-B.Z. thanks Y.-Z. Fan, Y.-Z. Wang, H. Wang, K. D. Alexander and D. Lazzati for helpful discussions. We are grateful to K. Hurley, I. Mitrofanov, A. Sanin, M. Litvak and W. Boynton for the use of Mars Odyssey data in the triangulation. We acknowledge the use of the public data from the Swift and Fermi data archives. B.-B.Z. and A.J.C.-T. acknowledge support from the Spanish Ministry Projects AYA2012-39727-C03-01 and AYA2015-71718-R. Part of this work made use of B.-B.Z.’s personal Interactive Data Language (IDL) code library ZBBIDL and personal Python library ZBBPY. The computation resources used in this work are owned by Scientist Support LLC. B.Z. acknowledges NASA NNX14AF85G and NNX15AK85G for support. Z.G.D. acknowledges the National Natural Science Foundation of China (NSFC) (grant 11573014). Y.-D.H. acknowledges support by China Scholarships Council (grant 201406660015). Mini-MegaTORTORA belongs to Kazan Federal University, and the work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. A.P., E.M., P.M. and A.V. are grateful to the Russian Foundation for Basic Research (grant 17-02-01388) for partial support. A.P. and S.B.P. acknowledge joint BRICS (Brazil, Russia, India, China and South Africa) grant RFBR 17-52-80139 and 388-ProFChEAP for partial support. R.I. is grateful to grant RUSTAVELI FR/379/6-300/14 for partial support. Observations on Mini-MegaTORTORA are supported by the Russian Science Foundation (grant 14-50-00043). A.V.F. and A.M. thank the Russian Science Foundation (grant 14-50-00043). L.M. and A.F.Z. acknowledge support from INTA-CEDEA ESAt personnel hosting the Pi of the Sky facility at the BOOTES-1 station. H.G. and X.-Y.W. acknowledge NSFC (grants 11603003 and 11625312, respectively). Z.G.D., X.-F.W., B.Z., X.-Y.W., L.S. and F.-W.Z. are also supported by the 973 program (grant 2014CB845800). F.-W.Z. is also supported in part by the NSFC (grants U1331101 and 11163003), the Guangxi Natural Science Foundation (grant 2013GXNSFAA019002) and the project of outstanding young teachers’ training in higher education institutions of Guangxi. L.S. acknowledges support by the NSFC (grant 11103083) and the Joint NSFC-ISF Research Program (grant 11361140349). S.O. acknowledges the support of the Leverhulme Trust. S.J. acknowledges support from Korea Basic Science Research Program through NRF-2014R1A6A3A03057484 and NRF-2015R1D1A4A01020961, and I.H.P. through NRF-2015R1A2A1A01006870 and NRF-2015R1A2A1A15055344. R.A., D.F. and D.S. acknowledge support from RSF (grant 17-12-01378). A.K. acknowledges the Science and Education Ministry of Kazakhstan (grant 0075/GF4).

Author information


  1. Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, Granada, Spain

    • B.-B. Zhang
    • , A. J. Castro-Tirado
    • , Y.-D. Hu
    • , S. Jeong
    • , R. Cunniffe
    • , J. C. Tello
    • , P. Ferrero
    •  & R. Sánchez-Ramírez
  2. School of Astronomy and Space Science, Nanjing University, Nanjing, China

    • B.-B. Zhang
    • , Z. G. Dai
    •  & X.-Y. Wang
  3. Scientist Support LLC, Madison, AL, USA

    • B.-B. Zhang
  4. Department of Physics and Astronomy, University of Nevada, Las Vegas, NV, USA

    • B. Zhang
  5. Department of Astronomy, School of Physics, Peking University, Beijing, China

    • B. Zhang
  6. Kavli Institute for Astron. Astrophys., Peking University, Beijing, China

    • B. Zhang
  7. Departamento de Ingeniería de Sistemas y Automática, Escuela de Ingenierías, Universidad de Málaga, Málaga, Spain

    • A. J. Castro-Tirado
  8. Key Laboratory of Modern Astron. Astrophys. (Nanjing University), Ministry of Education, Nanjing, China

    • Z. G. Dai
    •  & X.-Y. Wang
  9. School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai, China

    • P.-H. T. Tam
  10. Universidad de Granada, Facultad de Ciencias Campus Fuentenueva s/n, Granada, Spain

    • Y.-D. Hu
  11. Special Astrophysical Observatory of Russian Academy of Sciences, Nizhniy Arkhyz, Russia

    • S. Karpov
    • , G. Beskin
    • , A. Moskvitin
    • , T. Fatkhullin
    • , V. V. Sokolov
    •  & A. F. Valeev
  12. Kazan Federal University, Kazan, Russia

    • S. Karpov
    • , G. Beskin
    • , A. Biryukov
    • , V. Sasyuk
    •  & A. F. Valeev
  13. Space Research Institute of the Russian Academy of Sciences, Moscow, Russia

    • A. Pozanenko
    • , E. Mazaeva
    • , P. Minaev
    •  & A. Volnova
  14. National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russia

    • A. Pozanenko
  15. National Research University Higher School of Economics, Moscow, Russia

    • A. Pozanenko
  16. College of Science, Guilin University of Technology, Guilin, China

    • F.-W. Zhang
  17. Department of Physics, University of Warwick, Coventry, UK

    • S. Oates
  18. Department of Astronomy, Beijing Normal University, Beijing, China

    • H. Gao
  19. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China

    • X.-F. Wu
    •  & L. Shao
  20. School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui, China

    • X.-F. Wu
  21. Joint Center for Particle, Nuclear Physics and Cosmology, Nanjing University-Purple Mountain Observatory, Nanjing, China

    • X.-F. Wu
  22. Department of Space Sciences and Astronomy, Hebei Normal University, Shijiazhuang, China

    • L. Shao
  23. Department of Physics, Nanchang University, Nanchang, China

    • Q.-W. Tang
  24. Moscow State University, Moscow, Russia

    • A. Biryukov
  25. Research and Production Corporation ‘Precision Systems and Instruments’, Nizhniy Arkhyz, Russia

    • S. Bondar
    • , E. Ivanov
    • , E. Katkova
    • , N. Orekhova
    •  & A. Perkov
  26. Center for Theoretical Physics PAS, Warsaw, Poland

    • L. Mankiewicz
    •  & R. Opiela
  27. Faculty of Physics, University of Warsaw, Warsaw, Poland

    • A. F. Żarnecki
  28. National Centre for Nuclear Research, Warsaw, Poland

    • A. Cwiek
    •  & A. Zadrożny
  29. Ioffe Institute, Saint Petersburg, Russia

    • R. Aptekar
    • , D. Frederiks
    •  & D. Svinkin
  30. Fesenkov Astrophysical Institute, Almaty, Kazakhstan

    • A. Kusakin
  31. Kharadze Abastumani Astrophysical Observatory, Ilia State University, Tbilisi, Georgia

    • R. Inasaridze
  32. Ulugh Beg Astronomical Institute, Tashkent, Uzbekistan

    • O. Burhonov
  33. Crimean Astrophysical Observatory, Nauchny, Crimea, Russia

    • V. Rumyantsev
  34. Institute of Solar-Terrestrial Physics, PO Box 291, Irkutsk, Russia

    • E. Klunko
  35. Department of Physics, Sungkyunkwan University (SKKU), Suwon, Korea

    • S. Jeong
    •  & I. H. Park
  36. Astronomical Institute of the Academy of Sciences, Prague, Czech Republic

    • M. D. Caballero-García
    •  & M. Jelínek
  37. ARIES, Manora Peak, Nainital, India

    • S. B. Pandey
  38. School of Physics and Electronic Science, Guizhou Normal University, Guiyang, Guizhou, China

    • F. K. Peng
  39. Departamento de Algebra, Geometría y Topología, Facultad de Ciencias, Universidad de Málaga, Málaga, Spain

    • A. Castellón


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B.-B.Z. led the Fermi data analysis, modelling and physical explanations of GRB 160625B. B.Z., Z.G.D. and X.-Y.W. proposed the theoretical models to explain the data. B.-B.Z. and B.Z. wrote the manuscript. A.P. and P.M. participated in determination of the best spectral model for the sub-burst A and the physical model of the sub-burst B. P.-H.T.T., Y.-D.H. and F.-W.Z. helped with the data analysis and made the plots. A.J.C.-T., A.V., S.J. and S.K. worked on the Gran Telescopio CANARIAS photometry and spectroscopy. A.Ca., A.P., S.B., E.M., A.V., A.M. and V.S. reduced additional optical data. A.K., R.I., O.B., V.R., E.K. and A.M. conducted additional optical observations. G.B. and S.K. reduced the Mini-MegaTORTORA data. L.M., A.F.Z., A.Cw., R.O. and A.Z. worked on the Pi of the Sky data. All authors contributed to the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to B.-B. Zhang or B. Zhang.

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