Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory

TL;DR

This paper presents a combined fit of an astrophysical model to the energy spectrum and composition data of ultra-high-energy cosmic rays from the Pierre Auger Observatory, revealing source characteristics and the impact of uncertainties.

Contribution

It introduces a unified fitting approach to both spectrum and composition data, highlighting source properties and the influence of propagation uncertainties.

Findings

Sources have low maximum injection energies.

Sources exhibit hard spectra and heavy composition.

Propagation uncertainties significantly affect fit results.

Abstract

We present a combined fit of a simple astrophysical model of UHECR sources to both the energy spectrum and mass composition data measured by the Pierre Auger Observatory. The fit has been performed for energies above $5 \cdot 10^{18}$ eV, i.e.~the region of the all-particle spectrum above the so-called "ankle" feature. The astrophysical model we adopted consists of identical sources uniformly distributed in a comoving volume, where nuclei are accelerated through a rigidity-dependent mechanism. The fit results suggest sources characterized by relatively low maximum injection energies, hard spectra and heavy chemical composition. We also show that uncertainties about physical quantities relevant to UHECR propagation and shower development have a non-negligible impact on the fit results.