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Upper limit of the source distances
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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.