Constraining the sources of ultra-high-energy cosmic rays across and above the ankle with the spectrum and composition data measured at the Pierre Auger Observatory

TL;DR

This paper interprets Pierre Auger Observatory data on ultra-high-energy cosmic rays, modeling their spectrum and composition with a two-source astrophysical framework to understand their origins and the features observed at the ankle energy.

Contribution

It introduces a two-population astrophysical model that explains the spectrum and composition of UHECRs above 6×10^{17} eV, highlighting the role of mixed compositions and spectral features.

Findings

Sources above the ankle emit a mixed composition with a hard spectrum and low rigidity cutoff.

Below the ankle, the spectrum is very soft with a mix of protons and intermediate-mass nuclei.

The origin of intermediate-mass nuclei remains uncertain, possibly Galactic or extragalactic.

Abstract

In this work we present the interpretation of the energy spectrum and mass composition data as measured by the Pierre Auger Collaboration above $6 \times 10^{17}$ eV. We use an astrophysical model with two extragalactic source populations to model the hardening of the cosmic-ray flux at around $5\times 10^{18}$ eV (the so-called "ankle" feature) as a transition between these two components. We find our data to be well reproduced if sources above the ankle emit a mixed composition with a hard spectrum and a low rigidity cutoff. The component below the ankle is required to have a very soft spectrum and a mix of protons and intermediate-mass nuclei. The origin of this intermediate-mass component is not well constrained and it could originate from either Galactic or extragalactic sources. To the aim of evaluating our capability to constrain astrophysical models, we discuss the impact on the fit results of the main experimental systematic uncertainties and of the assumptions about quantities affecting the air shower development as well as the propagation and redshift distribution of injected ultra-high-energy cosmic rays (UHECRs).