Composition of cosmic rays accelerated in active galactic nuclei
Abstract
The composition of the overall spectrum of cosmic rays (CRs) is studied under the assumption that ultra high energy CRs above the energy 1017 eV are produced at the shock created by the expanding cocoons around active galactic nuclei (AGNs). It is shown that the expected CR composition is characterised by two peaks in the energy dependence of the mean CR atomic number <A(ε)>. The first one at the energy ε ≈ 1017 eV corresponds to the very end of the Galactic CR component, produced in supernova remnants (SNRs). It is followed by a sharp decrease of <A(ε)> within the energy interval from 1017 to 1018 eV. This is a signature of the transition from Galactic to extragalactic CRs. The second peak, with <ln A> ≈ 2, at energy ε≈ 1019 eV, expected at the beginning of the GZK cutoff, is the signature of the CR production by the nonrelativistic cocoon shocks. The calculated CR composition is consistent with the existing data. The alternative scenario, which suggests reacceleration increasing the energy of CRs produced in SNRs by a factor of 30, is also examined.