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 \Gamma into electron – positron pairs of similar Lorentz factor , while at the same time emitting most of the GRB luminosity at an energy E _ { p } \simeq 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 p \gamma \rightarrow pe ^ { - } e ^ { + } , lead to a maximum in the GRB luminosity at an energy E _ { p } \simeq 1 MeV , independent of the value of \Gamma .
Furthermore , the same threshold implies that the prompt \gamma - ray emission is only possible for \Gamma 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 \gamma - ray ( E \sim 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 \theta \simeq 1 / \Gamma 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 \Delta t \simeq 10 ^ { -2 } s , in agreement with observation .