The Imprint of Large Scale Structure on the Ultra-High-Energy Cosmic Ray Sky

TL;DR

This paper demonstrates that large-scale anisotropies in ultra-high-energy cosmic rays can be explained by their source distribution following local matter, providing insights into their origin, composition, and magnetic field interactions.

Contribution

It shows that matter distribution in the local universe can account for observed UHECR anisotropies without specifying production mechanisms, advancing understanding of their origins.

Findings

Good match with observed dipole anisotropy above 8×10^18 eV

Incompatibility of pure proton composition with observed anisotropy

Potential to constrain UHECR composition and magnetic fields

Abstract

Ultra-high-energy cosmic rays (UHECRs) are atomic nuclei from space with vastly higher energies than any other particles ever observed. Their origin and chemical composition remain a mystery. As we show here, the large- and intermediate-angular-scale anisotropies observed by the Pierre Auger Observatory are a powerful tool for understanding the origin of UHECRs. Without specifying any particular production mechanism, but only postulating that the source distribution follows the matter distribution of the local Universe, a good accounting of the magnitude, direction and energy dependence of the dipole anisotropy at energies above $8 \times 10^{18}$ eV is obtained, after taking into account the impact of energy losses during propagation (the "GZK horizon"), diffusion in extragalactic magnetic field and deflections in the Galactic magnetic field (GMF). This is a major step toward the long-standing hope of using UHECR anisotropies to constrain UHECR composition and magnetic fields. The observed dipole anisotropy is incompatible with a pure proton composition in this scenario. With a more accurate treatment of energy losses, it should be possible to further constrain the cosmic-ray composition and properties of the extragalactic magnetic field, self-consistently improve the GMF model, and potentially expose individual UHECR sources.