Observation of a Very Energetic Cosmic Ray Well Beyond the Predicted 2.7 K Cutoff in the Primary Energy Spectrum

Abstract
A very energetic cosmic ray of energy about (1.7-2.6) × 1020 eV was observed by the Akeno Giant Air Shower Array on 3 December 1993 from the direction of galactic longitude l=131° and galactic latitude b=41° within an error circle of 1.0° radius. If this cosmic ray were a proton; its origin could be extragalactic. However, the distance of the source cannot be much more than a few times 10 Mpc due to the energy loss during its travel from interactions with universal background radiation.

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