Ultra High Energy Cosmic Rays (uhecr)

Extremely energetic subatomic particles (mostly protons, but also some heavier atomic nuclei) with energies >109 MeV. The record holder so far is a UHECR with an energy of 6 × 1014 MeV! UHECRs are observed only in cosmic ray showers, because of their extremely low numbers. The source of UHECRs is a mystery, as is the mechanism that accelerates particles to these energies. They have sufficient energy to escape the typical magnetic field of a spiral galaxy, and most UHECRs are probably of extragalactic origin. Possible sources include active galactic nuclei, dormant quasars with associated supermassive black holes and galaxy mergers.