Ultra high-energy cosmic rays from beyond the Greisen-Zatsepin-Kuz'min horizon

TL;DR

This paper explores how ultra-high-energy cosmic rays from sources beyond the GZK horizon can significantly contribute to observed fluxes, especially considering heavy nuclei interactions, leading to an isotropic background component.

Contribution

It demonstrates the substantial contribution of distant sources beyond the GZK horizon to UHE CR flux, accounting for photo-spallation effects and evolving multi-particle horizons.

Findings

Distant sources beyond GZK horizon contribute significantly to UHE CR flux.

Heavy nuclear CRs enable a substantial isotropic background component.

CR horizons depend on nuclear composition and source evolution.

Abstract

Ultra-high-energy (UHE) cosmic rays (CRs) of energies $\sim (10^{18}-10^{20})~{\rm eV}$, accelerated in violent astrophysical environments, interact with cosmic background radiation fields via photo-hadronic processes, leading to strong attenuation. Typically, the Universe would become `opaque' to UHE CRs after several tens of Mpc, setting the boundary of the Greisen-Zatsepin-Kuz'min (GZK) horizon. In this work, we investigate the contribution of sources beyond the conventional GZK horizon to the UHE CR flux observed on Earth, when photo-spallation of the heavy nuclear CRs is taken into account. We demonstrate this contribution is substantial, despite the strong attenuation of UHE CRs. A significant consequence is the emergence of an isotropic background component in the observed flux of UHE CRs, coexisting with the anisotropic foreground component that are associated with nearby sources. Multi-particle CR horizons, which evolve over redshift, are determined by the CR nuclear composition. Thus, they are dependent on the source populations and source evolutionary histories.