Cosmic Rays from the Ankle to the Cut-Off

TL;DR

This paper reviews recent progress in understanding ultra-high-energy cosmic rays, focusing on the origin of flux suppression and the challenges in determining their composition and source models.

Contribution

It provides an overview of recent experimental and theoretical developments in cosmic ray research from the ankle to the highest energies, highlighting key open questions.

Findings

Flux suppression at ~4x10^{19} eV may be due to GZK-effect or source exhaustion.

High isotropy at the highest energies challenges proton-dominated models with weak magnetic fields.

Progress in measuring mass composition and magnetic field effects is crucial for future understanding.

Abstract

Recent advances in measuring and interpreting cosmic rays from the spectral ankle to the highest energies are briefly reviewed. The prime question at the highest energies is about the origin of the flux suppression observed at E ~ 4x10^{19} eV. Is this the long awaited GZK-effect or the exhaustion of sources? The key to answering this question will be provided by the largely unknown mass composition at the highest energies. The high level of isotropy observed even at the highest energies challenges models of a proton dominated composition if extragalactic magnetic fields are on the order of a few nG or less. We shall discuss the experimental and theoretical progress in the field and the prospects for the next decade.