A signature of EeV protons of Galactic origin

TL;DR

This paper explores the signatures of Galactic-origin protons at ultra-high energies around 1 EeV, highlighting a sharp flux increase and anisotropy that could be detected by current experiments, indicating a potential Galactic contribution to UHECRs.

Contribution

It introduces a numerical backtracking method to analyze the transition region of UHECRs, revealing characteristic flux and anisotropy features of Galactic protons.

Findings

Sharp flux increase across the transition energy

Strong anisotropy with a bright spot in the sky

A few percent Galactic proton admixture detectable

Abstract

We investigate signatures that would be produced in the spectrum and sky distribution of UHECR by a population of the Galactic sources of high-energy protons in the energy range around 1~EeV, i.e., around the diffusive-to-ballistic transition. In this regime, the CR flux has to be calculated numerically. We employ the approach that consists in backtracking anti-protons from Earth through the Galaxy and integrating the source emissivity along the trajectory. This approach makes evident two generic features of the transition region: sharp increase of the total flux as the energy decreases across the transition region, and its strong anisotropy (appearance of a bright compact spot) all the way until the onset of the diffusive regime. We then discuss and compare several methods to experimentally detect or constrain these features. We find that a few percent admixture of the Galactic protons can in principle be detected by the current UHECR experiments.