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

Abstract

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 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 Δt ≃ 10−2 s, in agreement with observation.