Upper limit of the source distances

Extremely energetic cosmic rays interact with the cosmic background photons via pair creation and photopion production and lose their energies during their trip. Therefore there is upper limit of distances which they can propagete in the space with a given energy. The above figure shows this limit (so called attenuation length) in case of cosmic ray protons. You see the 2x10^20 eV particles cannot propagate longer than 30 Mpc (100 million light years), which sets the limit concerning the location of possible sources.
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