Origin of ultra-high energy cosmic rays

Determine the astrophysical sources that produce ultra-high energy cosmic rays (UHECRs).

Background

The paper studies the Amaterasu event in the broader context of UHECR source identification. Despite extensive observational and modeling efforts, deflections in Galactic and extragalactic magnetic fields and limited statistics at the highest energies hinder definitive source associations.

This longstanding uncertainty motivates the paper’s simulation-based inference approach, which constrains potential source locations under realistic propagation effects, but the fundamental problem of identifying the classes of objects responsible for UHECR production remains unresolved.

References

The origin of these particles is still unknown despite numerous efforts to identify their sources.

— Beyond the Local Void: A comprehensive view on the origins of the Amaterasu particle  (2406.16483 - Bourriche et al., 2024) in Section 1. Introduction

More generally, the capability of Cygnus~A to accelerate particles up to the highest observed energies remains an open question.

— Cosmogenic neutrinos from FRII galaxies as potential origin of the ultra-high-energy KM3-230213A event  (2608.16540 - Allard et al., 17 Aug 2026) in Section 4, Summary and observational prospects, paragraph beginning “More generally, the capability of Cygnus A…”

The origin of UHECRs remains one of the most significant unresolved problems in modern astroparticle physics \citep{Kotera:2011cp}.

— Source models of ultrahigh-energy cosmic rays  (2608.27078 - Zhang, 27 Aug 2026) in Section 1, Introduction

An open question is whether the sources of UHECRs share similar properties.

— Source models of ultrahigh-energy cosmic rays  (2608.27078 - Zhang, 27 Aug 2026) in Section 2.1, “Hillas condition and energetics of UHECRs”

It remains an open question which sources are responsible for the cosmic ray flux between the ‘second knee’ spectral feature at 1017.5 eV [4–10], thought to be the cutoff of galactic cosmic rays, and the ‘ankle’ feature at 1018.6 eV [4, 11], presumed to indicate the dominance of extragalactic cosmic rays [3, 12].

— Exploring the sensitivity of in-ice radio detectors to cosmic ray mass composition  (2608.28536 - Alden et al., 28 Aug 2026) in Introduction

At present, the origin of ultra-high-energy ($\lesssim 10{19} \, \mathrm{eV}$) cosmic rays remains unknown: no suitable astrophysical accelerators are found in the local Universe, while cosmic rays with such extreme energies are unable to propagate over cosmological distances.

— Dark-matter relay for ultra-high-energy cosmic rays  (2609.26611 - Sekretov, 22 Sep 2026) in Section 1, Introduction