Composition of cosmic rays accelerated in active galactic nuclei

TL;DR

This paper models the composition of ultra high energy cosmic rays assuming they originate from shocks around active galactic nuclei, predicting characteristic peaks in atomic number that align with observational data.

Contribution

It introduces a model linking cosmic ray composition features to AGN cocoon shocks, providing a new explanation for observed peaks and transitions in cosmic ray spectra.

Findings

Two peaks in <A(ε)> at 10^{17} eV and 10^{19} eV

Transition signature from Galactic to extragalactic CRs

Consistency with existing cosmic ray composition data

Abstract

The composition of the overall spectrum of cosmic rays (CRs) is studied under the assumption that ultra high energy CRs above the energy 10^{17} 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(\epsilon)>. The first one at the energy \epsilon \approx 10^{17} 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(\epsilon)> within the energy interval from 10^{17} to 10^{18} eV. This is a signature of the transition from Galactic to extragalactic CRs. The second peak, with <ln A> \approx 2, at energy \epsilon\approx 10^{19} 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.