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.

  • Subscribe to Nature Astronomy for full access:



Additional access options:

Already a subscriber?  Log in  now or  Register  for online access.


  1. 1.

    Kumar, P. & Zhang, B. The physics of gamma-ray bursts & relativistic jets. Phys. Rep. 561, 1–109 (2015).

  2. 2.

    Ryde, F. et al. Identification and properties of the photospheric emission in GRB090902B. Astrophys. J. 709, L172–L177 (2010).

  3. 3.

    Band, D. et al. BATSE observations of gamma-ray burst spectra. I—spectral diversity. Astrophys. J. 413, 281–292 (1993).

  4. 4.

    Preece, R. et al. The first pulse of the extremely bright GRB 130427A: a test lab for synchrotron shocks. Science 343, 51–54 (2014).

  5. 5.

    Zhang, B.-B., Uhm, Z. L., Connaughton, V., Briggs, M. S. & Zhang, B. Synchrotron origin of the typical GRB Band function—case study of GRB 130606B. Astrophys. J. 816, 72 (2016).

  6. 6.

    Uhm, Z. L. & Zhang, B. Fast-cooling synchrotron radiation in a decaying magnetic field and γ-ray burst emission mechanism. Nat. Phys. 10, 351–356 (2014).

  7. 7.

    Zhang, B. & Pe’er, A. Evidence of an initially magnetically dominated outflow in GRB 080916C. Astrophys. J. 700, L65–L68 (2009).

  8. 8.

    Hascoët, R., Daigne, F., Mochkovitch, R. & Vennin, V. Do Fermi Large Area Telescope observations imply very large Lorentz factors in gamma-ray burst outflows? Mon. Not. R. Astron. Soc. 421, 525–545 (2012).

  9. 9.

    Axelsson, M. et al. GRB110721A: an extreme peak energy and signatures of the photosphere. Astrophys. J. 757, L31 (2012).

  10. 10.

    Gao, H. & Zhang, B. photosphere emission from a hybrid relativistic outflow with arbitrary dimensionless entropy and magnetization in GRBs. Astrophys. J. 801, 103 (2015).

  11. 11.

    Troja, E. et al. Significant and variable linear polarization during the prompt optical flash of GRB 160625B. Nature 547, 425–427 (2017).

  12. 12.

    Zhang, B. et al. Discerning the physical origins of cosmological gamma-ray bursts based on multiple observational criteria: the cases of z = 6.7 GRB 080913, z = 8.2 GRB 090423, and some short/hard GRBs. Astrophys. J. 703, 1696–1724 (2009).

  13. 13.

    Hu, Y.-D. et al. Internal energy dissipation of gamma-ray bursts observed with Swift: precursors, prompt gamma-rays, extended emission, and late X-ray flares. Astrophys. J. 789, 145 (2014).

  14. 14.

    Preece, R. D. et al. The BATSE gamma-ray burst spectral catalog. I. High time resolution spectroscopy of bright bursts using high energy resolution data. Astrophys J. Suppl. S. 126, 19–36 (2000).

  15. 15.

    Mészáros, P. & Rees, M. J. Steep slopes and preferred breaks in gamma-ray burst spectra: the role of photospheres and Comptonization. Astrophys. J. 530, 292–298 (2000).

  16. 16.

    Pe’er, A. Temporal evolution of thermal emission from relativistically expanding plasma. Astrophys. J. 682, 463–473 (2008).

  17. 17.

    Deng, W. & Zhang, B. Cosmological implications of fast radio burst/gamma-ray burst associations. Astrophys. J. 783, L35 (2014).

  18. 18.

    Murakami, T., Inoue, H., Nishimura, J., van Paradijs, J. & Fenimore, E. E. A gamma-ray burst preceded by X-ray activity. Nature 350, 592–594 (1991).

  19. 19.

    Pe’er, A., Ryde, F., Wijers, R. A. M. J., Mészáros, P. & Rees, M. J. A new method of determining the initial size and Lorentz factor of gamma-ray burst fireballs using a thermal emission component. Astrophys. J. 664, L1–L4 (2007).

  20. 20.

    Castro-Tirado, A. J. et al. GRB 021004: tomography of a gamma-ray burst progenitor and its host galaxy. Astron. Astrophys. 517, A61 (2010).

  21. 21.

    Racusin, J. L. et al. Broadband observations of the naked-eye γ-ray burst GRB080319B. Nature. 455, 183–188 (2008).

  22. 22.

    Piran, T., Sari, R. & Zou, Y.-C. Observational limits on inverse Compton processes in gamma-ray bursts. Mon. Not. R. Astron. Soc. 393, 1107–1113 (2009).

  23. 23.

    Zhang, B. & Yan, H. The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) model of gamma-ray bursts. Astrophys. J. 726, 90 (2011).

  24. 24.

    Ghirlanda, G., Celotti, A. & Ghisellini, G. Extremely hard GRB spectra prune down the forest of emission models. Astron. Astrophys. 406, 879–892 (2003).

  25. 25.

    Wang, X.-Y. & Mészáros, P. GRB precursors in the fallback collapsar scenario. Astrophys. J. 670, 1247–1253 (2007).

  26. 26.

    Proga, D. & Zhang, B. The late time evolution of gamma-ray bursts: ending hyperaccretion and producing flares. Mon. Not. R. Astron. Soc. 370, L61–L65 (2006).

  27. 27.

    Metzger, B. D., Giannios, D., Thompson, T. A., Bucciantini, N. & Quataert, E. The protomagnetar model for gamma-ray bursts. Mon. Not. R. Astron. Soc. 413, 2031–2056 (2011).

  28. 28.

    Komissarov, S. S. & Barkov, M. V. Magnetar-energized supernova explosions and gamma-ray burst jets. Mon. Not. R. Astron. Soc. 382, 1029–1040 (2007).

  29. 29.

    Burrows, D. N. et al. Bright X-ray flares in gamma-ray burst afterglows. Science 309, 1833–1835 (2005).

  30. 30.

    Ackermann, M. et al. Fermi-LAT observations of the gamma-ray burst GRB 130427A. Science 343, 42–47 (2014).

  31. 31.

    Ziaeepour, H., et al. Final Swift Observations of GRB 070721B GRB Coordinates Network Report 73 (2007).

  32. 32.

    Gruber, D. et al. Fermi/GBM observations of the ultra-long GRB 091024. A burst with an optical flash. Astron. Astrophys. 528, A15 (2011).

  33. 33.

    Zhang, B.-B. et al. Unusual central engine activity in the double burst GRB 110709B. Astrophys. J. 748, 132 (2012).

  34. 34.

    Abdo, A. A. et al. Fermi observations of GRB 090902B: a distinct spectral component in the prompt and delayed emission. Astrophys. J. 706, L138–L144 (2009).

  35. 35.

    Burlon, D. et al. Precursors in swift gamma ray bursts with redshift. Astrophys. J. 685, L19 (2008).

  36. 36.

    Troja, E., Rosswog, S. & Gehrels, N. Precursors of short gamma-ray bursts. Astrophys. J. 723, 1711–1717 (2010).

  37. 37.

    Lazzati, D. Precursor activity in bright, long BATSE gamma-ray bursts. Mon. Not. R. Astron. Soc. 357, 722–731 (2005).

  38. 38.

    Zhang, B.-B. et al. A comprehensive analysis of Fermi gamma-ray burst data. I. Spectral components and the possible physical origins of LAT/GBM GRBs. Astrophys. J. 730, 141 (2011).

  39. 39.

    Zhang, B.-B. et al. An analysis of Chandra deep follow-up gamma-ray bursts: implications for off-axis jets. Astrophys. J. 806, 15 (2015).

  40. 40.

    Cash, W. Parameter estimation in astronomy through application of the likelihood ratio. Astrophys. J. 228, 939–947 (1979).

  41. 41.

    Tang, S. M. & Zhang, S. N. Time lag between prompt optical emission and γ-rays in GRBs. Astron. Astrophys. 456, 141–143 (2006).

  42. 42.

    Beskin, G. et al. Fast optical variability of a naked-eye burst—manifestation of the periodic activity of an internal engine. Astrophys. J. 719, L10–L14 (2010).

  43. 43.

    Li, Z. & Waxman, E. Prompt optical emission from residual collisions in gamma-ray burst outflows. Astrophys. J. 674, L65 (2008).

  44. 44.

    Fan, Y.-Z., Zhang, B. & Wei, D.-M. Naked-eye optical flash from gamma-ray burst 080319B: tracing the decaying neutrons in the outflow. Phys. Rev. D. 79, 021301 (2009).

Download references


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-Ramrez
  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


  1. Search for B.-B. Zhang in:

  2. Search for B. Zhang in:

  3. Search for A. J. Castro-Tirado in:

  4. Search for Z. G. Dai in:

  5. Search for P.-H. T. Tam in:

  6. Search for X.-Y. Wang in:

  7. Search for Y.-D. Hu in:

  8. Search for S. Karpov in:

  9. Search for A. Pozanenko in:

  10. Search for F.-W. Zhang in:

  11. Search for E. Mazaeva in:

  12. Search for P. Minaev in:

  13. Search for A. Volnova in:

  14. Search for S. Oates in:

  15. Search for H. Gao in:

  16. Search for X.-F. Wu in:

  17. Search for L. Shao in:

  18. Search for Q.-W. Tang in:

  19. Search for G. Beskin in:

  20. Search for A. Biryukov in:

  21. Search for S. Bondar in:

  22. Search for E. Ivanov in:

  23. Search for E. Katkova in:

  24. Search for N. Orekhova in:

  25. Search for A. Perkov in:

  26. Search for V. Sasyuk in:

  27. Search for L. Mankiewicz in:

  28. Search for A. F. Żarnecki in:

  29. Search for A. Cwiek in:

  30. Search for R. Opiela in:

  31. Search for A. Zadrożny in:

  32. Search for R. Aptekar in:

  33. Search for D. Frederiks in:

  34. Search for D. Svinkin in:

  35. Search for A. Kusakin in:

  36. Search for R. Inasaridze in:

  37. Search for O. Burhonov in:

  38. Search for V. Rumyantsev in:

  39. Search for E. Klunko in:

  40. Search for A. Moskvitin in:

  41. Search for T. Fatkhullin in:

  42. Search for V. V. Sokolov in:

  43. Search for A. F. Valeev in:

  44. Search for S. Jeong in:

  45. Search for I. H. Park in:

  46. Search for M. D. Caballero-García in:

  47. Search for R. Cunniffe in:

  48. Search for J. C. Tello in:

  49. Search for P. Ferrero in:

  50. Search for S. B. Pandey in:

  51. Search for M. Jelínek in:

  52. Search for F. K. Peng in:

  53. Search for R. Sánchez-Ramrez in:

  54. Search for A. Castellón in:


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.

Electronic supplementary material

  1. Supplementary Information

    Supplementary Table I–V, Supplementary Figure 1–8 and Supplementary References