Ultrahigh-Energy Cosmic Rays: New View From 0° to 80°
The nature and origin of ultrahigh-energy cosmic rays (UHECRs)—charged particles with energies ranging from approximately 1018eV to 1020eV—remain a significant challenge in astrophysics, according to a review published in August 2025 in the Annual Review of Astronomy and Astrophysics.
These particles, exhibiting energies inaccessible to terrestrial accelerators, present fundamental physics questions and necessitate a strong connection between theoretical and observational astronomy. Researchers Noémie Globus and Roger D. Blandford detail recent progress in determining the composition and sources of UHECRs, noting that the highest-energy events appear to be heavy nuclei with rigidity extending up to approximately 1018 eV.
A key observation is a significant dipole anisotropy in UHECR arrival directions, detected at a 6.9σ confidence level. However, interpreting this anisotropy is complicated by the limited understanding of Galactic magnetic fields. The observed luminosity density of UHECRs is approximately 1044 erg Mpc-3 year-1, a value that places constraints on potential origin scenarios.
The review identifies diffusive shock acceleration and unipolar induction as the most promising acceleration mechanisms for these particles. Potential sources include intergalactic accretion shocks and relativistic jets emanating from both stellar-mass and supermassive black holes. The study highlights the potential for multimessenger astronomy—combining observations across electromagnetic radiation, neutrinos, and gravitational waves—to resolve the UHECR riddle.
The study builds on decades of research into UHECRs, dating back 60 years to their initial discovery. Previous work, including a 2011 review by Kumiko Kotera and Angela V. Olinto, emphasized the potential of UHECR studies to constrain Galactic and extragalactic magnetic field structures and to probe particle interactions at energies far exceeding those achievable in terrestrial laboratories. Kotera and Olinto’s work also suggested a likely extragalactic origin for these particles, with candidate sources ranging from compact object formation to gamma-ray bursts and active galactic nuclei.
Understanding the propagation of UHECRs from cosmological distances is crucial, involving interactions with the cosmic background radiation and magnetic fields. The observed hint of sky anisotropies and unexpected evolution of composition indicators continue to drive research in this field. Future observations, aiming for increased sensitivity, may enable charged particle astronomy and the detection of ultrahigh-energy photons and neutrinos, offering further insights into the universe at extreme energies.
The Annual Review article notes that the study of UHECRs can also constrain the structure of the Galactic and extragalactic magnetic fields and probe particle interactions at energies orders of magnitude higher than achieved in terrestrial accelerators.