A Possible Nearby Origin for the Highest-Energy Events Observed

Abstract

The detection of ultra high energy cosmic ray (UHECR) events beyond the cuto. expected from proton interactions with the microwave background has continued to defy an explanation on simple terms. Assuming that powerful radio galaxies are the source of UHECR’s at E ≥ 1019 eV, and that these are protons, susceptible to photon interactions over propagation paths longer than ≃ 30 Mpc, we present a possible solution that explains both the puzzling lack of a GZK cutoff due to photon interactions and the observed energy-independent isotropy of UHECR arrival directions. Our model has three essential ingredients: a) The highest energy particles come from the nearest powerful radio galaxy M87, b) they are isotropized in the magnetic field of a halowind of our Galaxy, and c) this isotropization can be naturally explained by a k -2 wavenumber spectrum of magnetic irregularities in the halo wind, which makes the bending of particle orbits independent of energy. All three concepts are well established, and in essence we have combined them. At progressively lower energies near 5 × 1019 eV, some UHECR arrivals from additional radio galaxies at moderate distances will also get mixed in. Of all proposed solutions to the challenge presented by the high energy CR events this is the simplest one, and is it based on known physical processes.