Transition from Fireball to Poynting-flux-dominated Outflow in Three-Episode GRB 160625B

• Published in: arXiv.org 2017-10
• Main Authors: B -B Zhang, Zhang, B, Castro-Tirado, A J, Dai, Z G, Tam, P -H T, X -Y Wang, Y -D Hu, Karpov, S, Pozanenko, A, F -W Zhang, Mazaeva, E, Minaev, P, Volnova, A, Oates, S, Gao, H, X -F Wu, Shao, L, Q -W Tang, Beskin, G, Biryukov, A, Bondar, S, Ivanov, E, Katkova, E, Orekhova, N, Perkov, A, Sasyuk, V, Mankiewicz, L, Zarnecki, A F, Cwiek, A, Opiela, R, Zadrozny, 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-Garcia, M D, Cunniffe, R, Tello, J C, Ferrero, P, Pandey, S B, Jelinek, M, Peng, F K, Sanchez-Ramirez, R, Castellon, A
• Format: Article
• Language: English
• Subjects: Composition; Ejecta; Emission; Flux; Gamma ray bursts; Gamma rays; Luminosity; Outflow; Spectra; Synchrotron radiation; Thermal radiation
• ISSN: 2331-8422

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.