Astroparticle Physics

86 6 Primary Cosmic RaysFig. 6.7Sketch of proton and iron-nucleustrajectories in our Milky Way at10 18 eVlight of charged particles and photons, one must take intoaccount that charged particles are delayed with respect tophotons because they travel on trajectories bent by the magneticfield.Since the magnetic deflection is proportional to thecharge of a particle, proton astronomy is more promisingthan astronomy with heavy nuclei. This idea is outlined inFig. 6.7 where the trajectories of protons and an iron nucleus(Z = 26) at an energy of 10 18 eV are sketched for ourgalaxy. This figure clearly shows that one should only use –if experimentally possible – protons for particle astronomy.Actually, there are some hints that the origins of the fewevents with energies > 10 19 eV could lie in the supergalacticplane, a cluster of relatively close-by galaxies includingour Milky Way (‘local super galaxy’). It has also been discussedthat the galactic center of our Milky Way and in particularthe Cygnus region could be responsible for a certainanisotropy at 10 18 eV. It must, however, be mentioned thatclaims for such a possible correlation are based on very lowstatistics and are therefore not unanimously supported. Theycertainly need further experimental confirmation.6.2 Neutrino Astronomy“Neutrino physics is largely an art oflearning a great deal by observing nothing.”Haim Hararilimits of classical astronomyThe disadvantage of classical astronomies like observationsin the radio, infrared, optical, ultraviolet, X-ray, or γ -rayband is related to the fact that electromagnetic radiationis quickly absorbed in matter. Therefore, with these astronomiesone can only observe the surfaces of astronomicalobjects. In addition, energetic γ rays from distant sourcesare attenuated via γγ interactions with photons of the blackbodyradiation by the processγ + γ → e + + e − .Energetic photons (> 10 15 eV), for example, from the LargeMagellanic Cloud (LMC, 52 kpc distance) are significantlyattenuated by this process (see Fig. 4.9).Charged primaries can in principle also be used in as-troparticle physics. However, the directional informationcharged primaries