Ultra-High Energy Cosmic Rays: Results and Prospects

TL;DR

Recent advancements in cosmic ray observations have revealed new spectral features and raised questions about their origins, emphasizing the importance of composition and magnetic fields in understanding ultra-high energy cosmic rays.

Contribution

This paper reviews recent experimental and theoretical progress in ultra-high energy cosmic ray research and discusses future prospects for the field.

Findings

Detection of a second knee dominated by heavy primaries

Observation of an ankle-like feature in the light component

Challenges to proton-dominated models due to isotropy data

Abstract

Observations of cosmic rays have been improved at all energies, both in terms of higher statistics and reduced systematics. As a result, the all particle cosmic ray energy spectrum starts to exhibit more structures than could be seen previously. Most importantly, a second knee in the cosmic ray spectrum -- dominated by heavy primaries -- is reported just below 10^{17} eV. The light component, on the other hand, exhibits an ankle like feature above 10^{17} eV and starts to dominate the flux at the ankle. The key question at the highest energies is about the origin of the flux suppression observed at energies above 5 x 10^{19} eV. Is this the long awaited GZK-effect or the exhaustion of sources? The key to answering this question is again given by the still largely unknown mass composition at the highest energies. Data from different observatories don't quite agree and common efforts have been started to settle that question. The high level of isotropy observed even at the highest energies starts to challenge a proton dominated composition if extragalactic (EG) magnetic fields are on the order of a few nG or more. We shall discuss the experimental and theoretical progress in the field and the prospects for the next decade.