Extremely high energy cosmic rays

• Published in: Nuclear physics. Section B, Proceedings supplement Proceedings supplement, 2000, Vol.80 (1), p.33-40
• Main Author: Cronin, James W.
• Format: Article
• Language: English
• ISSN: 0920-5632; 1873-3832
• DOI: 10.1016/S0920-5632(99)00827-0

The evidence for the existence of cosmic rays with energies in excess of 10 20 eV is now overwhelming. There is so far no indication of the GZK cutoff in the energy spectrum at 5 × 10 19 eV. This conclusion is not firm for lack of statistics. A cutoff would be expected if the sources of the cosmic rays were distributed uniformly throughout the cosmos. The sources of cosmic rays with energy above the GZK cutoff must be at a distance ≤ 100 Mpc, and if they are protons they are very likely to point to these sources. There are no easy explanations how known astrophysical objects can accelerate protons (or atomic nuclei) to these energies. This difficulty has led to speculation that there may be exotic sources such as topological defects which produce these energetic cosmic rays directly along with a copious supply of neutrinos of similar energy. The fluxes of these cosmic rays is very low and large instruments are required to observe them even with modest statistics. One such instrument, the Pierre Auger Observatory, is described. It is designed for all-sky coverage and the construction of its southern site will begin in Argentina in 1999.