Ultra High Energy Cosmic Ray Source Models: Successes, Challenges and General Predictions

TL;DR

This review discusses the observational constraints, source models, and future prospects for understanding the origins and acceleration mechanisms of Ultra High Energy Cosmic Rays, highlighting the importance of advanced detection arrays.

Contribution

It provides a comprehensive overview of current source models, observational constraints, and emphasizes the need for larger arrays to identify cosmic ray sources.

Findings

Heavier nuclei dominate above 5 EeV

Cosmic rays are strongly suppressed above 50 EeV

Dipole anisotropy is the main departure from isotropy

Abstract

Understanding the acceleration of Ultra High Energy Cosmic Rays is one of the great challenges of contemporary astrophysics. In this short review, we summarize the general observational constraints on their composition, spectrum and isotropy which indicate that nuclei heavier than single protons dominate their spectra above $\sim 5\,{\rm EeV}$, that they are strongly suppressed above energies $\sim50\,{\rm EeV}$, and that the only significant departure from isotropy is a dipole. Constraints based upon photopion and photodisintegration losses allow their ranges and luminosity density to be estimated. Three general classes of source model are discussed - magnetospheric models (including neutron stars and black holes), jet models (including Gamma Ray Bursts, Active Galactic Nuclei and Tidal Disruption Events) and Diffusive Shock Acceleration models (involving large accretion shocks around rich clusters of galaxies). The value of constructing larger and more capable arrays to measure individual masses at the highest energies and probably identifying their sources is emphasized.