The “Supercritical Pile” Model of GRB: Thresholds, Polarization, Time Lags

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) of 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 Ep ≃ 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 Ep ≃ 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 ≃ 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 ∆t ≃ 10−2 s, in agreement with observation.