Coronal Gamma Ray Bursts as the sources of Ultra High Energy Cosmic Rays?

Abstract

I consider the possibility that Ultra High Energy Cosmic Rays are accelerated in Gamma Ray Bursts located in the Galactic corona, thus circumventing the problem raised by Greisen--Zatsepin--Kuz'min cutoff. The acceleration of UHECRs could occur in the pulsars which, in the coronal GRB model, produce them: the same parameters that permit fitting GRBs' observations in the model of Podsiadlowski, Rees and Ruderman (1995) lead to an estimate of the highest achievable energies corresponding to that of the Bird et al (1994) event, and to very low luminosities in cosmic rays. I show that, if the observations of Milgrom and Usov (1995a) are confirmed, the extragalactic GRBs' model for the acceleration of UHECRs is untenable, but the same constraint does not apply to the coronal model. Also, I show that the efficiency of particle acceleration needs be much smaller (and less demanding) than in cosmological models of GRBs. Uncertainties remain about the ensuing cosmic ray spectral distribution. I also briefly discuss observational strategies to distinguish between the two possibilities.