28th International Cosmic Ray Conference 651 Is the HiRes Energy Spectrum Really Consistent with GZK Cutoff?

Ultra-high energy cosmic ray (UHECR) protons produced by uniformly distributed astrophysical sources contradict the energy spectrum measured by both the AGASA and HiRes experiments, assuming the small scale clustering of UHECR observed by AGASA is caused by point-like sources. In that case, the small number of sources leads to a sharp exponential cutoff at the energy E < 1020 eV in the UHECR spectrum [5]. The HiRes experiment published recently its data from monocular ob-servations [1]. They showed that the UHECR flux is consistent with the GZK cutoff expected for uniformly, continuously distributed sources. As a result, the simplest model of UHECR—protons accelerated in uniformly distributed, extra-galactic sources—seems to be a convincing explanation of their data. The authors of Ref. [2] found as fingerprints of the expected interactions of UHE protons with CMB photons a dip at E ∼ 1×1019 eV, a bump and the beginning of the cutoff in the measured spectra of four UHECR experiments. The agreement of the spectral shape calculated for protons with the measured spectra is excellent, apart from an excess in the AGASA data above E ≥ 8 × 1019 eV. These findings point to an AGN origin of UHECR below E ≤ 1020 eV and to protons as primaries. Despite the fact that the AGASA experiment sees a significant number of events above the GZK cutoff [8], the model of proton primaries from extragalactic sources looks very attractive, because it does not require new physics. The model of uniformly, continuously distributed sources is based on the assumption that the number of UHECR sources is so large that a significant fraction of sources is inside the GZK volume. However, as it was shown in