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

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${}_{\mathrm{p},\mathrm{iso}}\sim 4\times 10^{53}$ 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.

arXiv commentRevised version reflecting the referees' comments. 27 pages, 11 figures, 5 tables. The final edited version will appear in Nature Astronomy

Cited by in corpus (9)

• Consistency with synchrotron emission in the bright GRB 160625B observed by Fermi
• A New Test of Lorentz Invariance Violation: the Spectral Lag Transition of GRB 160625B
• Theoretical Description Of GRB 160625B with Wind-to-ISM Transition and Implications for a Magnetized Outflow
• On the origin of the multi-GeV photons from the closest burst with intermediate luminosity: GRB 190829A
• The Optical/NIR afterglow of GRB 111209A: Complex yet not Unprecedented
• Model-independent constraints on Lorentz invariance violation: implication from updated Gamma-ray burst observations
• Constraining Anisotropic Lorentz Violation via the Spectral-Lag Transition of GRB 160625B
• Modeling the high-energy emission in GRB 110721A and implications on the early multiwavelength and polarimetric observations
• A Low-latency Pipeline for GRB Light Curve and Spectrum using Fermi/GBM Near Real-time Data