We present the general notions and observational consequences of the “ Supercritical Pile ” GRB model ; the fundamental feature of this model is a detailed process for the conversion of the energy stored in relativistic protons in the GRB Relativistic Blast Waves ( RBW ) into relativistic electrons and then into radiation .
The conversion is effected through the p \gamma \rightarrow p e ^ { + } e ^ { - } reaction , whose kinematic threshold is imprinted on the GRB spectra to provide a peak of their emitted luminosity at energy E _ { p } \sim 1 MeV in the lab frame .
We extend this model to include , in addition to the ( quasi– ) thermal relativistic post-shock protons an accelerated component of power law form .
This component guarantees the production of e ^ { + } e ^ { - } - pairs even after the RBW has slowed down to the point that its ( quasi– ) thermal protons can not fulfill the threshold of the above reaction .
We suggest that this last condition marks the transition from the prompt to the afterglow GRB phase .
We also discuss conditions under which this transition is accompanied by a significant drop in the flux and could thus account for several puzzling , recent observations .
Finally , we indicate that the same mechanism applied to the late stages of the GRB evolution leads to a decrease in E _ { p } \propto \Gamma ^ { 2 } ( t ) \propto t ^ { -3 / 4 } , a feature amenable to future observational tests .