On the acceleration of Ultra High Energy Cosmic Rays in Gamma Ray Bursts

UHECRs are roughly isotropic and attain very large energies, E ∼> 3×10 20 eV . Conventional models fail to explain both facts. I show here that acceleration of UHECRs in GRBs satisfies both observational constraints. Using Mészáros and Rees’ (1994) model of GRBs as due to hyperrelativistic shocks, I show that the highest energies that can be attained thusly are E ≃ 10 θ−5/3n 1 eV , explaining the energy of the Bird et al. (1995) event even without beaming. The traditional photopion catastrophe affecting UHECR acceleration in AGNs is circumvented. An order of magnitude estimate shows that the total energy flux of UHECRs at the Earth is also correctly reproduced. A test of the model based upon the UHECRs’ distribution upon the plane of the sky is briefly discussed. Subject headings: acceleration of particles – gamma-rays: bursts