Implications of the cosmic ray spectrum for the mass composition at the highest energies

TL;DR

This paper reviews how ultra-high-energy cosmic rays interact with radiation fields, affecting their composition and flux, and discusses implications for the observed mass composition at the highest energies.

Contribution

It demonstrates that the propagated cosmic-ray composition at Earth can be dominated by heavy nuclei like Fe, depending on source characteristics and maximum proton energies.

Findings

Propagated composition can be Fe or sub-Fe dominated at highest energies.

Spectrum alone does not constrain source composition.

Flux above 3×10^{20} eV is reduced if protons are limited at sources.

Abstract

The significant attenuation of the cosmic-ray flux above $\sim 5 10^{19}$ eV suggests that the observed high-energy spectrum is shaped by the so-called GZK effect. This interaction of ultra-high-energy cosmic rays (UHECRs) with the ambient radiation fields also affects their composition. We review the effect of photo-dissociation interactions on different nuclear species and analyze the phenomenology of secondary proton production as a function of energy. We show that, by itself, the UHECR spectrum does not constrain the cosmic-ray composition at their extragalactic sources. While the propagated composition (i.e., as observed at Earth) cannot contain significant amounts of intermediate mass nuclei (say between He and Si), whatever the source composition, and while it is vastly proton-dominated when protons are able to reach energies above $10^{20}$ eV at the source, we show that the propagated composition can be dominated by Fe and sub-Fe nuclei at the highest energies, either if the sources are very strongly enriched in Fe nuclei (a rather improbable situation), or if the accelerated protons have a maximum energy of a few $10^{19}$ eV at the sources. We also show that in the latter cases, the expected flux above $3 10^{20}$ eV is very much reduced compared to the case when protons dominate in this energy range, both at the sources and at Earth.