Enhancing the Relative Fe-to-Proton Abundance in Ultra-High-Energy Cosmic Rays

TL;DR

This paper investigates how a distribution of maximum energies among sources influences the composition and spectrum of ultra-high-energy cosmic rays, highlighting a significant enhancement of heavy nuclei like iron at high energies.

Contribution

It introduces a model where sources have varying maximum proton energies, showing this leads to increased heavy nuclei abundance and impacts UHECR spectrum interpretation.

Findings

Heavy nuclei, especially iron, are enhanced by factors of 2-10 at lower energies.

At the highest energies, heavy nuclei enhancement ratios increase further.

Source energy distribution affects both the spectrum and composition of observed UHECRs.

Abstract

We study a generic class of models for ultra-high energy cosmic ray (UHECR) phenomenology, in which the sources accelerate protons and nuclei with a power-law spectrum having the same index, but with different values for the maximum proton energies, distributed according to a power-law. We show that, for energies sufficiently lower than the maximum proton energy, such models are equivalent to single-type source models, with a larger effective power law index and a heavier composition at the source. We calculate the resulting enhancement of the abundance of nuclei, and find typical values of a factor 2-10 for Fe nuclei. At the highest energies, the heavy nuclei enhancement ratios become larger, and the granularity of the sources must also be taken into account. We conclude that the effect of a distribution of maximum energies among sources must be considered in order to understand both the energy spectrum and the composition of UHECRs, as measured on Earth.