11.2. UCHERs - Ultra High Energy Cosmic Rays

Waxman [294] and Vietri [295] have shown that the observed flux of UCHERs (above 10¹⁹ eV) is consistent with the idea that these are produced by the fireball shocks provided that f_UCHERs- 1. Milgrom & Usov [296] pointed out that the error boxes of the two highest energy UCHERs contain strong GRBs - suggesting an association between the two phenomena. The relativistic fireball shocks that appear in GRBs are among the few astronomical objects that satisfy the conditions for shock acceleration of UCHERs. Waxman [298] has shown that the spectrum of UCHERs is consistent with the one expected from Fermi acceleration within those shocks.