The Astrophysics of Ultrahigh Energy Cosmic Rays

TL;DR

This paper reviews the current understanding of ultrahigh energy cosmic rays, discussing their potential sources, propagation effects, and the prospects for future observations to unveil their origins and the physics at extreme energies.

Contribution

It provides a comprehensive overview of the astrophysical sources, propagation mechanisms, and observational challenges of ultrahigh energy cosmic rays, highlighting new questions and future research directions.

Findings

Spectrum consistent with extragalactic sources

Hints of sky anisotropies observed

Potential to probe fundamental particle interactions

Abstract

The origin of the highest energy cosmic rays is still unknown. The discovery of their sources will reveal the workings of the most energetic astrophysical accelerators in the universe. Current observations show a spectrum consistent with an origin in extragalactic astrophysical sources. Candidate sources range from the birth of compact objects to explosions related to gamma-ray bursts or to events in active galaxies. We discuss the main effects of propagation from cosmologically distant sources including interactions with cosmic background radiation and magnetic fields. We examine possible acceleration mechanisms leading to a survey of candidate sources and their signatures. New questions arise from an observed hint of sky anisotropies and an unexpected evolution of composition indicators. Future observations may reach the necessary sensitivity to achieve charged particle astronomy and to observe ultrahigh energy photons and neutrinos, which will further illuminate the workings of the universe at these extreme energies. In addition to fostering a new understanding of high-energy astrophysical phenomena, the study of ultrahigh energy cosmic rays can constrain the structure of the Galactic and extragalactic magnetic fields as well as probe particle interactions at energies orders of magnitude higher than achieved in terrestrial accelerators.