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

Understand

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.