Cosmic-ray particle with a kinetic energy greater than 1 EeV
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018electronvolts, approximately 0.16 joules),[1] far beyond both the rest mass and energies typical of other cosmic ray particles. The origin of these highest energy cosmic ray is not known.[2]
These particles are extremely 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 3,000 km2 (1,200 sq mi) area surveyed by the observatory.[3]