Possible Clustering of the Most Energetic Cosmic Rays within a Limited Space Angle Observed by the Akeno Giant Air Shower Array

We report the detection of a 51-joule (320 +/- 90 EeV) cosmic ray by the Fly's Eye air shower detector in Utah. This is substantially greater than the energy of any previously reported cosmic ray. A Greisen-Zatsepin-Kuz'min cutoff of the energy spectrum (due to pion photoproduction energy losses) should occur below this energy unless the highest energy cosmic rays have traveled less than about 30 Mpc. The error box for the arrival direction in galactic coordinates is centered on b=9.6 deg, l=163.4 deg. The particle cascade reached a maximum size near a depth of 815 g/cm^2 in the atmosphere, a depth which does not uniquely identify the type of primary particle.

In this Letter we examine the arrival directions of the most energetic cosmic rays (E > 2 * 10^19 eV) detected by several air shower experiments. We find that data taken by different air shower arrays show positive correlations, indicating a non--uniform arrival direction distribution. We also find that the events with energy $ > 4 * 10^19 eV exhibit a correlation with the general direction of the supergalactic plane, where a large number of potential sources is located. If confirmed by data from other experiments our results would support models for the extragalactic origin of the highest energy cosmic rays.