example, to AGNs or to young pulsar magnetospheres. In a $10^7$ K radiation field around a young neutron star TeV protons can propagate only a few km, so cannot escape the magnetosphere or be accelerated to still higher energies (Venkatesan, 1997). Similarly the radiation field near an accreting massive black hole in an AGN will limit the escape energy of protons to less than a few hundred TeV. In either case, accelerator regions further out from the central massive object remain viable by this condition.

Models of Gamma Ray Bursts (GRBs) typically call for very strong magnetic fields in equipartition with the radiation field (e.g., Mészáros & Rees 1994). Then synchrotron losses are particularly important and can provide a strong limitation to the energies of escaping particles, which may still come close enough to ZeV to make them interesting. Adiabatic losses would also be severe if the acceleration took place only at very early times in the expansion of the fireball, although conditions may be suitable long enough to avoid this issue.

At the other extreme of source environments, photopion energy losses probably eliminate cosmic structure shocks as viable candidates for UHECRs, at least above the GZK feature (Norman, 1995; 