Composition of UHECR and the Pierre Auger Observatory Spectrum

TL;DR

This study analyzes the Pierre Auger ultra-high energy cosmic ray spectrum assuming pure compositions like protons, oxygen, or iron, and finds multiple compatible source models, including iron-dominated scenarios, consistent with recent composition measurements.

Contribution

It provides a comprehensive scan of source parameters and demonstrates that the spectrum can be explained by different compositions, highlighting the viability of iron-dominated sources.

Findings

Spectrum fits with pure compositions are possible.

Two distinct maximum energy solutions for iron sources.

Iron-dominated models align with recent composition hints.

Abstract

We fit the recently published Pierre Auger ultra-high energy cosmic ray spectrum assuming that either nucleons or nuclei are emitted at the sources. We consider the simplified cases of pure proton, or pure oxygen, or pure iron injection. We perform an exhaustive scan in the source evolution factor, the spectral index, the maximum energy of the source spectrum Z E_{max}, and the minimum distance to the sources. We show that the Pierre Auger spectrum agrees with any of the source compositions we assumed. For iron, in particular, there are two distinct solutions with high and low E_{max} (e.g. 6.4 10^{20} eV and 2 10^{19} eV) respectively which could be distinguished by either a large fraction or the near absence of proton primaries at the highest energies. We raise the possibility that an iron dominated injected flux may be in line with the latest composition measurement from the Pierre Auger Observatory where a hint of heavy element dominance is seen.