THE ENERGY SPECTRUM OBSERVED BY THE AGASA EXPERIMENT AND THE SPATIAL DISTRIBUTION OF THE SOURCES OF ULTRA-HIGH-ENERGY COSMIC RAYS

摘要

Seven and a half years of continuous monitoring of giant air showers triggered by ultra-high-energy cosmic rays have been summarized recently by the AGASA collaboration. The resulting energy spectrum indicates clearly that the cosmic-ray spectrum extends well beyond the Greisen-Zatsepin-Kuzmin (GZK) cutoff at ～5 x 10~19 eV. Furthermore, despite the small-number statistics involved, some structure in the spectrum may be emerging. Using numerical simulations, it is demonstrated in the present work that these features are consistent with a spatial distribution of sources that follows the distribution of luminous matter in the local universe. Therefore, from this point of view, there is no need for a second high-energy component of cosmic rays dominating the spectrum beyond the GZK cutoff.