LIMITS ON SOURCE DISTANCES FOR THE MOST ENERGETIC COSMIC-RAYS

The propagation of the highest energy cosmic rays through the 2.7 K mi crowave background is considered. Photo-pion production interactions c ause energy losses for primary cosmic ray protons which result in sign ificant energy degradation over intergalactic distances. The process o f energy loss is discussed and an estimate is made of the average dist ances travelled for observed cosmic rays with a range of energies, ass uming a power law source spectrum. At energies over 10(20) eV, the ave rage distances travelled-are a few tens of megaparsecs, limiting possi ble sources to the volume dominated by the supergalactic plane.