Astroparticle Physics

162 7 Secondary Cosmic Raysextensive air showersand γ -ray burstsenergies > 10 20 TeVparticle astronomy?coincidencesover large distances(≈ 6 × 10 19 eV). Protons of this energy would point backto the sources, because galactic and intergalactic magneticfields only cause angular distortions on the order of onedegree at these high energies. The irregularities of magneticfields, however, could lead to significant time delays betweenneutrinos and photons on one hand and protons, onthe other hand, from such distant sources. This comes aboutbecause the proton trajectories are somewhat longer, eventhough their magnetic deflection is rather small. Dependingon the distance from the source, time delays of months andeven years can occur. This effect is of particular importance,if γ -ray bursters are also able to accelerate the highestenergyparticles and if one wants to correlate the arrivaltimes of photons from γ -ray bursts with those of extensiveair showers initiated by charged primaries.The few measured particles with energies in excess of10 20 eV show a non-uniform distribution with a certain clusteringnear the local supergalactic plane. The fact that theattenuation length of protons with energies > 10 20 eV inthe intergalactic space is approximately 10 Mpc would makean origin in the local supercluster (maximum size 30 Mpc)plausible.Out of the six measured showers with primary energyexceeding 10 20 eV the directions of origin for two eventsare identical within the measurement accuracy. This directioncoincides with the position of a radio galaxy (3C134),whose distance unfortunately is unknown, since it lies in thedirection of the galactic plane, where optical measurementsof extragalactic objects are difficult because of interstellarabsorption. The coincidence of the radio galaxy 3C134 withthe arrival directions of the two highest-energy particles can,of course, also be an accident.Normal extensive air showers have lateral widths of atmost 10 km, even at the highest energies. However, thereare indications that correlations between arrival times of airshowers over distances of more than 100 km exist. Such coincidencescould be understood by assuming that energeticprimary cosmic particles undergo interactions or fragmentationsat large distances from Earth. The secondary particlesproduced in these interactions would initiate separateair showers in the atmosphere (Fig. 7.32).Even moderate distances of only one parsec (3×10 16 m)are sufficient to produce separations of air showers at Earthon the order of 100 km (primary energy 10 20 eV, transverse