What do we know about cosmic rays with energies above 5 EeV?

TL;DR

Cosmic rays above 5 EeV show anisotropy indicating extragalactic origins, with models reproducing flux features and composition changes, yet their sources remain largely unidentified despite recent anisotropy signals.

Contribution

This paper synthesizes observational data and models to elucidate the origins, composition, and spectral features of ultra-high-energy cosmic rays above 5 EeV.

Findings

Detection of large-scale anisotropy correlating with extragalactic sources.

Reproduction of flux spectrum features including the ankle, flux suppression, and spectral instep.

Increasing average atomic mass with energy indicating composition changes.

Abstract

Cosmic rays begin to reveal their secrets at energies above 5 EeV. Beyond this characteristic energy, known as the spectral "ankle", the arrival-direction data from the Pierre Auger Observatory show anisotropy on large angular scales of increasing amplitude with energy. This discovery provides observational evidence that cosmic rays beyond the ankle originate outside the Milky Way, as expected from the weak Galactic confinement and the high luminosity required for the sources. Synthetic models of extragalactic source populations emitting fully ionized atoms have allowed us to reproduce the cosmic-ray flux beyond the ankle for almost a decade. These models capture the various slope breaks in the spectrum at ultra-high energies, including the flux suppression at ${\sim}\,$45 EeV and the recently measured feature at ${\sim}\,$15 EeV, known as the spectral "instep". Such slope breaks are understood as changes in nuclear composition, with the average atomic mass increasing with energy. The population of astrophysical sources responsible for accelerating these nuclei remains unidentified, although serious contenders have been identified. Particularly instructive are the latest searches at the highest energies for anisotropies correlated with the flux patterns expected from galaxies outside the Local Group, which are approaching $5\,\sigma$.