The essence of the "Supercritical Pile" model is a process for converting the energy stored in the relativistic protons of a Relativistic Blast Wave (RBW) with Lorentz factor Γ into electron — positron pairs of similar Lorentz factor, while at the same time emitting most of the GRB luminosity at an energy E_p approx= 1 MeV. This is achieved by scattering the synchrotron radiation emitted by the RBW in an upstream located "mirror" and then re-intercepting it by the RBW. The repeated scatterings of radiation between the RBW and the "mirror", along with the threshold of the pair production reaction pγ → pe~–e~+, lead to a maximum in the GRB luminosity at an energy E_p approx= 1 MeV, independent of the value of Γ. Furthermore, the same threshold implies that the prompt γ-ray emission is only possible for Γ larger than a minimum value, thereby providing a "natural" account for the termination of this stage of the GRB as the RBW slows down. Within this model the γ-ray (E ~ 100 keV – 1 MeV) emission process is due to Inverse Compton scattering and it is thus expected to be highly polarized if viewed at angles θ approx= 1/Γ to the RBW's direction of motion. Finally, the model also predicts lags in the light curves of the lower energy photons with respect to those of higher energy; these are of purely kinematic origin and of magnitude Δt approx= 10–2 s, in agreement with observation.
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