Transition from fireball to Poynting-flux-dominated outflow in the three-episode GRB 160625B

Publication date: 
Main author: 
Zhang B.-B.
IAA authors: 
Zhang, B.-B.;Castro-Tirado, A.J.;Hu, Y.-D.;Jeong, S.;Cunniffe, R.;Tello, J.C.;Ferrero, P.;Sánchez-Ramrez, R.
Zhang B.-B., Zhang B., Castro-Tirado A.J., Dai Z.G., Tam P.-H.T., Wang X.-Y., Hu Y.-D., Karpov S., Pozanenko A., Zhang F.-W., Mazæva E., Minæv P., Volnova A., Oates S., Gao H., Wu X.-F., Shao L., Tang Q.-W., Beskin G., Biryukov A., Bondar S., Ivanov E., Katkova E., Orekhova N., Perkov A., Sasyuk V., Mankiewicz L., Żarnecki A.F., Cwiek A., Opiela R., Zadrożny A., Aptekar R., Frederiks D., Svinkin D., Kusakin A., Inasaridze R., Burhonov O., Rumyantsev V., Klunko E., Moskvitin A., Fatkhullin T., Sokolov V.V., Valeev A.F., Jeong S., Park I.H., Caballero-García M.D., Cunniffe R., Tello J.C., Ferrero P., Pandey S.B., Jelínek M., Peng F.K., Sánchez-Ramrez R., Castellón A.
Nature Astronomy
Publication type: 
The ejecta composition is an open question in gamma-ray burst (GRB) physics 1 . Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis 2, suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the Band function 3-5, consistent with a synchrotron radiation origin 5,6 and suggesting that the jet is Poynting-flux dominated at the central engine and probably in the emission region as well 7,8 . There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component 9, suggesting a probable hybrid jet composition 10 . 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 L p,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. © 2017 The Author(s).
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