โ Scribed by A.M. Hillas
No coin nor oath required. For personal study only.
The geometrical arguments that most UHE cosmic rays (below โผ 10 20 eV) come from very distant sources are rehearsed. It is argued that because concentrated release of gravitational energy, needed to power the accelerators, decays over long periods, one can hardly escape the expectation that the "ankle" in the cosmic-ray spectrum is twofold, being partly due to the effect of serious energy losses, through electron-pair production, of UHE cosmic rays generated long ago. In support of this interpretation, the well-measured cosmic ray spectrum below 10 19 eV can be analysed into three components -cosmic rays of normal galactic composition from diffusive shock acceleration in supernova remnants, whose spectrum turns down rather sharply near 3 PeV rigidity, a second rigidity-dependent component extending to around 10 17 eV rigidity, which may be due to special supernova remnants, and an extragalactic component dominated by hydrogen and helium. The expected high ridge of the latter flux, somewhat below 10 18 ev, before serious losses have set in, then accounts for the considerable fraction of protons reported in cosmic rays at that energy, well before the "ankle". Thus, even before the main GZK effect occurs, 85% of UHE cosmic rays have already been lost, mainly to electron pair production, confirming one aspect of the energy problem for GRBs as sources. However, radio galaxy sources offer no easy solution, as the magnetic fields required to accelerate particles to extreme energies are more problematic than often assumed.
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