Experimental constraints on the astrophysical interpretation of the cosmic ray Galactic-extragalactic transition region

TL;DR

This paper uses a diffusion model to analyze the transition region between Galactic and extragalactic cosmic rays, constraining their spectra and composition to match observations from HiRes and Auger.

Contribution

It introduces a detailed analysis of the Galactic-extragalactic transition using a diffusion model, highlighting the need for additional high-energy components beyond standard supernova remnants.

Findings

A mixed extragalactic spectrum is favored.

Additional high-energy Galactic components are necessary.

The composition of these components varies with energy.

Abstract

The energy region spanning from $\sim 10^{17}$ to $\lesssim 10^{19}$ eV is critical for understanding both, the Galactic and the extragalactic cosmic ray fluxes. This is the region where the propagation regime of nuclei inside the Galactic magnetic environment changes from diffusive to ballistic, as well as the region where, very likely, the most powerful Galactic accelerators reach their maximum output energies. In this work, a diffusion Galactic model is used to analyze the end of the Galactic cosmic ray spectrum and its mixing with the extragalactic cosmic ray flux. In particular, we study the conditions that must be met, from the spectral and composition points of view, by the Galactic and the extragalactic fluxes in order to reproduce simultaneously the total spectrum and elongation rate measured over the transition region by HiRes and Auger. Our analysis favors a mixed extragalactic spectrum in combination with a Galactic spectrum enhanced by additional high energy components, i.e., extending beyond the maximum energies expected from regular supernova remnants. The two additional components have mixed composition, with the lowest energy one heavier than the highest energy one. The potential impact on the astrophysical analysis of the assumed hadronic interaction model is also assessed in detail.