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

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.; Mazaeva  E.; Minaev  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.

doi:DOI: 10.1038/s41550-017-0309-8;eprintid: arXiv:1612.03089

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

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.; Mazaeva E.; Minaev 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.

Nature Astronomy, 2018, 2: 69-75.

DOI: DOI: 10.1038/s41550-017-0309-8;eprintid: arXiv:1612.03089

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Summary

The ejecta composition is an open question in gamma-ray burst (GRB)physics1. Some GRBs possess a quasi-thermal spectralcomponent in the time-resolved spectral analysis2, suggestinga hot fireball origin. Others show a featureless non-thermal spectrumknown as the Band function3-5, consistent with a synchrotronradiation origin5,6 and suggesting that the jet isPoynting-flux dominated at the central engine and probably in theemission region as well7,8. There are also bursts showing asub-dominant thermal component and a dominant synchrotroncomponent9, suggesting a probable hybrid jetcomposition10. Here, we report an extraordinarily bright GRB160625B, simultaneously observed in gamma-ray and optical wavelengths,whose prompt emission consists of three isolated episodes separated bylong quiescent intervals, with the durations of each sub-burst beingapproximately 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 detailedtime-resolved spectral analysis in each episode, from precursor to mainburst and to extended emission. The spectral properties of the first twosub-bursts are distinctly different, allowing us to observe thetransition from thermal to non-thermal radiation between well-separatedemission episodes within a single GRB. Such a transition is a clearindication of the change of jet composition from a fireball to aPoynting-flux-dominated jet.

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