Extragalactic cosmic rays and their signatures

Abstract

The signatures of UHE proton propagation through CMB are pair-production dip and GZK cutoff. The visible manifestations of these spectral features are ankle, beginning of GZK cutoff in the differential spectrum and E_{1/2} in integral spectrum. Observed in all experiments, the ankle is usually interpreted as transition from galactic to extragalactic cosmic rays. Using the mass composition measured by HiRes, Telescope Array (TA) and Auger detectors at energy (1-3) EeV, calculated anisotropy of galactic cosmic rays at these energies, and the elongation curves we strongly argue against the interpretation of the ankle given above. The transition must occur at lower energy, most probably at the second knee as the dip model predicts. The other prediction of this model, the shape of the dip, is well confirmed by HiRes, TA, AGASA and Yakutsk detectors, and, after recalibration of energies, by Auger detector. Predicted beginning of GZK cutoff and E_{1/2} agree well with HiRes and TA data. However, directly measured mass composition remains a puzzle. While HiRes and TA detectors observe the proton-dominated mass composition, as required by the dip model, the data of Auger detector strongly evidence for nuclei mass composition becoming steadily heavier at energy higher than 4 EeV and reaching Iron at energy about 35 EeV. The Auger-based scenario is consistent with another interpretation of the ankle at energy E_a=4 EeV as transition from extragalactic protons to extragalactic nuclei. The heavy- nuclei dominance at higher energies may be provided by low-energy of acceleration for protons E_{max} \sim 4 EeV and rigidity-dependent E_{max}^A =Z E_{max}$ for nuclei. The highest energy suppression may be explained as nuclei-destroying cutoff.