The origin of the highest energy cosmic rays

Abstract

The recent detection of several cosmic ray particle events with energies beyond eV is challenging current astrophysical theories. The microwave background interacts with high-energy nuclei and protons and thus does not allow particles beyond eV to reach us from cosmological distances. However, we now have a significant number of clear events of particles with energies beyond this limit. These highest energy cosmic rays cannot be contained in our Galactic disc and therefore ought to originate further outside. In this review we first discuss the boundary conditions for theories to account for these very high-energy particles. The most stringent conditions arise from the space and time possible in any particular scenario for the acceleration, since (a) the Larmor radius scale has to fit clearly inside the system under consideration and (b) the time scale for acceleration needs to be actually available. Then we proceed to discuss the various proposals to explain these high-energy events. Many possible sites for their origin have been proposed, such as the decay of exotic particles, shocks in the large scale structure of the universe, compact objects, Gamma-ray bursts, large scale shocks in our Galactic halo, clusters of galaxies, active galactic nuclei, and, specifically, radio galaxies. This last theory has been worked out in most detail, and therefore ought to be refutable or confirmable in the near future. In any model in which the cosmic rays arrive from nearby cosmological distances, say, from sources related to galaxies, we can make some strong predictions. The clustering of arrival directions in the sky ought to correspond to the source clustering for energies at which intergalactic scattering by magnetic fields is no longer important, and for which the cosmologically local structure of the universe is still inhomogeneous. Above eV the arrival directions of cosmic rays, as seen by the Haverah Park and Akeno arrays, are no longer isotropic, but appear to partially cluster towards the supergalactic plane, the locus of cosmologically nearby normal galaxies, and radio galaxies. Some local enhancements of the very high-energy cosmic rays may be due to several identifiable radio galaxies; one such candidate is the radio galaxy 3C134. As a corollary we describe a minimalist theory for the origin of magnetic fields in galaxies, deriving them from magnetic stellar winds and stellar dynamos. If the correlation with the supergalactic plane, based here on the Haverah Park and Akeno data, can be verified with the data from future air shower experiments, it would provide strong support for the existence of very energetic nuclei, most likely cosmic ray protons, in sources that cluster cosmologically just as galaxies and radio galaxies do.