Abstract:

We propose that the internal energy of the gamma-ray burst (GRB) blast waves, thought to be stored in the form of relativistic protons comoving with the blast wave, is converted explosively (i.e., on light crossing timescales) into relativistic electrons of the same Lorentz factor, which are responsible for the production of observed prompt γ-ray emission of the burst. This conversion is the result of the combined effects of the reflection of photons produced within the flow by upstream located matter, their reinterception by the blast wave, and their eventual conversion into e⁺e^- pairs in interactions with the relativistic protons of the blast wave (via the pγ-->pe⁺e^- reaction). This entire procedure is contingent on two conditions on the relativistic protons: a kinematic one imposed by the threshold of the pγ-->pe⁺e^- reaction and a dynamic one related to the column density of the postshock matter to the same process. This latter condition is in essence identical to that of the criticality of a nuclear pile, hence the terminology. It is argued that the properties of relativistic blast waves operating under these conditions are consistent with GRB phenomenology, including the recently found correlation between quiescence periods and subsequent flare fluence. Furthermore, it is shown that, when operating near threshold, the resulting GRB spectrum produces its peak luminosity at an energy (in the lab frame) E~=m_ec², thereby providing an answer to this outstanding question of GRBs.

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