THE ORIGIN OF COSMIC-RAYS ABOVE 10(18.5) EV

We discuss the origin of ultra-high-energy cosmic rays (UHECRs) in the energy range above 10(18.5) eV where it is indicated that the spectru m becomes protonic and extends smoothly up to at least 10(19.5) eV and is consistent with a smooth extension to 10(20.5) eV. The acceleratio n of the 10(19.5) eV component must occur within similar to 1 Gpc. We rule out the production and escape of protons and neutrons from active galactic nuclei. Composition arguments make unlikely any origin in me tal-rich environments such as rich clusters and the inner regions of g alaxies. We dismiss the canonical extended halo models since such halo s are almost never observed although diffuse halos have been seen in Q SO absorption-line studies of metallic absorption lines. Large-scale s hocks from explosions and winds are analyzed including those originati ng both recently and at earlier cosmological epochs. Large-scale shock s can work well only if they occur in microgauss fields. Hot spots and cocoons of radio sources are a plausible source for UHECRs with the p rincipal uncertainty in both cases being the adopted or inferred magne tic held strength. Cosmic shacks are formed as structure develops duri ng gravitational collapse of primordial perturbations such as are foun d in the standard cosmological models in, for example, both pancakes a nd collisions of hierarchical merging subunits. Cosmic shocks can be g ood sites for UHECR acceleration if there is a primordial field of ord er greater than or equal to 10(-9) G or, again, if microgauss fields c an be self-generated in shocks. Table 1 summarizes the results of our analysis of all the conventional possibilities and indicates that jets , radio source cocoons, structure formation in clusters superclusters, and large-scale structures are all reasonable sources for the product ion of UHECRs. We examine the possibility of a second-stage diffuse ac celeration process from an ensemble of shocks that might explain the h ighest energy particles above 100 EeV by boosting their energy from 30 to 300 EeV into a diffuse isotropic component. Table 2 summarizes our results and indicates that at the present time there is as yet no suc h candidate ensemble of shocks that has both the power and the volume- filling factor to achieve this effect. The highest energy cosmic rays must therefore have been accelerated in one stage from their parent ob ject.