Where do ultra-high-energy cosmic rays come from?
Charged particles detected above 10^18 eV — including events near 10^20 eV, macroscopic energy in a single subatomic particle — must be accelerated by the most extreme objects in the universe. Candidate sources include AGN jets, gamma-ray bursts and magnetars, but deflection by magnetic fields scrambles the directions.
Why it is hard
Charged nuclei are deflected by intergalactic and Galactic magnetic fields we barely know. Composition measurements are model-dependent (light protons vs. heavy nuclei behave very differently in transit), and the flux above the GZK cutoff is a few particles per square kilometer per century.
Current evidence
Auger and Telescope Array see a flux suppression at the expected GZK energy and a ~7% dipole anisotropy above 8 EeV, plus hints of correlation with nearby starburst galaxies and possibly Cen A — but no source has been unambiguously identified.
Possible approaches
Larger exposure (AugerPrime, GRAND, POEMMA-class space missions), composition measurements near the ankle, improved Galactic magnetic-field maps from Faraday rotation surveys, and neutrino/gamma-ray multimessenger coincidences.
Related topics
Sources
- The Pierre Auger Observatory — Pierre Auger Observatory
- Research Highlights — IceCube Neutrino Observatory
- What Are Active Galactic Nuclei? — NASA Science
- CTAO Technology — How CTAO works — Cherenkov Telescope Array Observatory