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

• DOI: 10.1038/s41550-017-0309-8
• Publication Date: 2017-11-16T16:42:56Z
• Authors: B.-B. Zhang; B. Zhang; A. J. Castro-Tirado; Z. G. Dai; P.-H. T. Tam; X.-Y. Wang; Y.-D. Hu; S. Karpov; A. Pozanenko; F.-W. Zhang; E. Mazaeva; P. Minaev; A. Volnova; S. Oates; H. Gao; X.-F. Wu; L. Shao; Q.-W. Tang; G. Beskin; A. Biryukov; S. Bondar; E. Ivanov; E. Katkova; N. Orekhova; A. Perkov; V. Sasyuk; L. Mankiewicz; A. F. Żarnecki; A. Cwiek; R. Opiela; A. Zadrożny; R. Aptekar; D. Frederiks; D. Svinkin; A. Kusakin; R. Inasaridze; O. Burhonov; V. Rumyantsev; E. Klunko; A. Moskvitin; T. Fatkhullin; V. V. Sokolov; A. F. Valeev; S. Jeong; I. H. Park; M. D. Caballero-G...; R. Cunniffe; J. C. Tello; P. Ferrero; S. B. Pandey; M. Jelínek; F. K. Peng; R. Sánchez-Ramírez; A. Castellón
• Keywords: High-energy astrophysics; High Energy Astrophysical Phenomena (astro-ph.HE); 13. Climate action; 4. Education; 0103 physical sciences; FOS: Physical sciences; Astrophysics - High Energy Astrophysical Phenomena; Stars; 01 natural sciences; 7. Clean energy

Revised version reflecting the referees' comments. 27 pages, 11 figures, 5 tables. The final edited version will appear in Nature Astronomy<br/>The ejecta composition is an open question in gamma-ray bursts (GRB) physics. Some GRBs possess a quasi-thermal spectral component in the time-resolved spectral analysis, suggesting a hot fireball origin. Others show a featureless non-thermal spectrum known as the "Band" function, consistent with a synchrotron radiation origin and suggesting that the jet is Poynting-flux-dominated at the central engine and likely in the emission region as well. There are also bursts showing a sub-dominant thermal component and a dominant synchrotron component suggesting a likely hybrid jet composition. Here we report an extraordinarily bright GRB 160625B, simultaneously observed in gamma-rays and optical wavelengths, whose prompt emission consists of three isolated episodes separated by long quiescent intervals, with the durations of each "sub-burst" being $\sim$ 0.8 s, 35 s, and 212 s, respectively. Its high brightness (with isotropic peak luminosity L$_{\rm p, iso}\sim 4\times 10^{53}$ erg/s) 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.<br/>