The Mystery of Ultra-High Energy Cosmic Rays

TL;DR

This paper reviews the challenges and current understanding of the origins of ultra-high-energy cosmic rays above 10^20 eV, highlighting the difficulties in identifying their sources and the potential for future research to solve this cosmic mystery.

Contribution

It provides a concise overview of theoretical models and discusses future observatory prospects for understanding ultra-high-energy cosmic rays.

Findings

Isotropy suggests extragalactic origin

Photon-pion production limits sources to within 50 Mpc

Current models face challenges due to large gyroradii and energy losses

Abstract

The origin of cosmic rays with energies higher than 10$^{20}$ eV remains a mystery. Accelerating particles up to these energies is a challenge even for the most energetic astrophysical objects known. While the isotropy in arrival directions argues for an extra-galactic origin, the photon-pion production off the cosmic background radiation limits the sources of such particles to systems less than 50 Mpc away from us. The combination of large gyroradii, efficient energy losses, and isotropic arrival directions defies most of the proposed astrophysical accelerators as well as the more exotic alternatives. I briefly review theoretical models for the acceleration and propagation of ultra-high-energy cosmic-rays and discuss the potential for future observatories to resolve this cosmic mystery.