Ultra-high-energy cosmic ray

Cosmic-ray particle with a kinetic energy above 1 EeV

In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray particles. The origin of these highest energy cosmic rays is not known. These particles are rare; between 2004 and 2007, the initial runs of the Pierre Auger Observatory (PAO) detected 27 events with estimated arrival energies above 5.7×1019 eV, that is, about one such event every four weeks in the 3000 km² (1200 mi²) area surveyed by the observatory.

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