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The physical processes of the gamma-ray emission and particle accelerationduring the prompt phase in GRBs are still unsettled. In order to perform anunambiguous physical modelling of observations, a clear identification of theemission mechanism is needed. An instance of a clear identification is thesynchrotron emission during the very strong flare in GRB160821A, that occursduring the prompt phase at 135 s. Here we show that the distribution of theradiating electrons in this flare is initially very narrow, but later developsa power-law tail of accelerated electrons. We thus identify for the first timethe onset of particle acceleration in a GRB jet. The flare is consistent with alate energy release from the central engine causing an external-shock as itencounters a preexisting ring nebula of a progenitor Wolf-Rayet star.Relativistic forward and reverse shocks develop, leading to two distinctemission zones with similar properties. The particle acceleration only occursin the forward shock, moving into the dense nebula matter. Here, themagnetisation also decreases below the critical value, which allows for Fermiacceleration to operate. Using this fact, we find a bulk Lorentz factor of $420\simleq \Gamma \simleq 770$, and an emission radius of $R \sim 10^{18}$ cm,indicating a tenuous gas of the immediate circumburst surrounding. Theobservation of the onset of particle acceleration thus gives new andindependent constraints on the properties of the flow as well as on theories ofparticle acceleration in collisionless astrophysical shocks.

Onset of particle acceleration during the prompt phase in gamma-ray bursts as revealed by synchrotron emission in GRB160821A