Transition from Fireball to Poynting-flux-dominated Outflow in Three-Episode GRB 160625B
astro-ph.HE
/ 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
and 44 more authors
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. Zarnecki, A. Cwiek, R. Opiela, A. Zadrozny, R. Aptekar, D. Frederiks, D. Svinkin
/ Abstract
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.