C. Cosmic Rays and Ultra High Energy Cosmic Rays

Already in 1990 Shemi and Piran [385] noticed that a fireball may produce cosmic rays. However the flux of "low" energy (up to 10¹⁴ eV) that they considered was smaller by several orders of magnitude than the observed flux of cosmic rays that are accelerated in SNRs. Hence this component isn't important.

Waxman [429] and independently Vietri [422] noticed that protons can be accelerated up to 10²⁰ eV within the relativistic shocks that take place in GRBs. Namely internal shocks or the reverse shock in GRBs are among the few locations in the Universe where the shock acceleration condition (Eq. 14 needed to accelerate protons up to 10²⁰ eV, the Hillas criterion can be satisfied. Moreover to within an order of magnitude the flux of -rays reaching earth from GRBs is comparable to the observed flux of UHECRs (Ultra High Energy Cosmic Rays) [429]. Thus, if GRBs produce a comparable energy in -rays and in UHECRs they could be the source of the highest energy Cosmic rays.

Greisen [146] and Zatsepin and Kuzmin [445] (GZK) pointed out that the highest energy CR (above 10^19.5 eV) are attenuated as they propagate via the Cosmic Microwave background (CMBR). This happens because at this high energies the protons can interact with the CMBR photons and produce pions. The typical mean free path of a ultra high energy proton in the CMBR decreases rapidly with energy and for a 10²⁰ eV proton it is only several tens Mpc. Thus, the observed UHECRs at energies above the GZK energy (~ 10^19.5 eV must arrive from relatively nearby (on cosmological scale) sources. However, there are no known steady state sources within this distance (see however Farrar and Piran [89]). GRBs as a transient phenomenon could be a "hidden" source of UHECRs. There won't be a direct association between GRBs and arrival of UHECRs as the later are deflected by the intergalactic magnetic field. This leads to an angular deflection as well as a long time delay. If GRBs are sources of UHECRs then we expect a break in the UHECR spectrum at the GZK energy - below the GZK energy we will detect UHECRs from the whole universe. Above the GZK energy we will detect only "local" UHECRs from within the nearest several dozen Mpc. Bahcall and Waxman [13] suggested that recent observations imply that such a break has been seen. However, the observational situation is not clear as yet and a final resolution would most likely require the Auger UHECR detector.