Astroparticle Physics

7.5 Nature and Origin of the Highest-Energy Cosmic Rays 163primordialparticlecorrelated air showersat Earthinteractionin the galaxyFig. 7.32Possible explanation forcorrelations between distantextensive air showersmomenta ≈ 0.3GeV/c). Variations in arrival times of theseshowers could be explained by unequal energies of the fragmentswhich could cause different propagation times. Galacticor extragalactic magnetic fields could also affect the trajectoriesof the fragments in a different way thus also influencingthe arrival times.correlated showers7.5 Nature and Origin of the Highest-EnergyCosmic Rays“The universe is full of magical thingspatiently waiting for our wits to growsharper.”Eden PhillpottsAs already explained in Sect. 7.4, the highest-energy particlesof cosmic rays appear to be of extragalactic origin.The problem of the sources of these particles is closely relatedto the identity of these particles. Up to the present timeone had always assumed that the chemical composition ofprimary cosmic rays might change with energy. However,one always anticipated that the highest-energy particles wereeither protons, light, or possibly medium heavy nuclei (upto iron). For particles with energies exceeding 10 20 eV thisproblem is completely open. In the following the candidateswhich might be responsible for cosmic-ray events with energies> 10 20 eV will be critically reviewed.Up to now only a handful of events with energies exceeding10 20 eV have been observed. Due to the measurementtechnique via extensive-air-shower experiments theenergy assignments are connected with an experimental errorof typically ±30%. For the accelerated parent particlesidentityof high-energy primaries?events with E>10 20 eV