Acceleration of UHE Cosmic Rays in Gamma-Ray Bursts

Abstract

Gamma-Ray Bursts are good candidates of the ``bottom up'' scenario for the generation of the UHE Cosmic Rays. In the most discussed model of GRBs, namely the ``fireball'' model, a highly relativistic shock forms and seems capable of accelerating the cosmic rays up to the EeV range. However, only the first Fermi cycle produces a large energy gain to particles coming from the external medium. Thus, a complementary acceleration is proposed, downstream of the external shock, in the relativistic plasma of the GRBs, where crossings of relativistic fronts are likely to occur. Both forward and backward fronts are necessary for the internal Fermi acceleration to work and the physical process that generates them is presented. We found that there exists a relevant physical process similar to Brillouin backscattering that redistributes the incoming energy in the plasma shell. This redistribution occurs through the generation of sound waves that heat the plasma shell and also through the generation of both forward and backward relativistic Alfven fronts that accelerate cosmic rays by the Fermi process. We show that this ensemble of processes is able to account for the generation of UHE cosmic rays. There are two opportunities for these combined processes, first during the ``primary'' Gamma-Ray Burst where baryons are entrained by the relativistic pair wind, second during the predeceleration and deceleration stages when the fireball interacts with the interstellar medium. However, because of the synchrotron losses, only the second stage can produce the UHE cosmic rays.