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

A very energetic cosmic ray of energy about (1.7-2.6) \ifmmode\times\else\texttimes\fi{} ${10}^{20}$ eV was observed by the Akeno Giant Air Shower Array on 3 December 1993 from the direction of galactic longitude $l=131\ifmmode^\circ\else\textdegree\fi{}$ and galactic latitude $b=\ensuremath{-}41\ifmmode^\circ\else\textdegree\fi{}$ within an error circle of 1.0\ifmmode^\circ\else\textdegree\fi{} 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.