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The ejecta composition is an open question in gamma-ray bursts (GRB) physics.Some GRBs possess a quasi-thermal spectral component in the time-resolvedspectral analysis, suggesting a hot fireball origin. Others show a featurelessnon-thermal spectrum known as the "Band" function, consistent with asynchrotron radiation origin and suggesting that the jet isPoynting-flux-dominated at the central engine and likely in the emission regionas well. There are also bursts showing a sub-dominant thermal component and adominant synchrotron component suggesting a likely hybrid jet composition. Herewe report an extraordinarily bright GRB 160625B, simultaneously observed ingamma-rays and optical wavelengths, whose prompt emission consists of threeisolated episodes separated by long quiescent intervals, with the durations ofeach "sub-burst" being $\sim$ 0.8 s, 35 s, and 212 s, respectively. Its highbrightness (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 eachepisode, from precursor to main burst and to extended emission. The spectralproperties of the first two sub-bursts are distinctly different, allowing us toobserve the transition from thermal to non-thermal radiation betweenwell-separated emission episodes within a single GRB. Such a transition is aclear indication of the change of jet composition from a fireball to aPoynting-flux-dominated jet.

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