Anisotropy and chemical composition of ultra-high energy cosmic rays using arrival directions measured by the Pierre Auger Observatory

TL;DR

This study investigates the anisotropy and chemical composition of ultra-high energy cosmic rays, revealing that the absence of expected anisotropies at lower energies constrains the proton fraction in cosmic ray sources.

Contribution

It provides new constraints on the proton component of cosmic rays by analyzing anisotropy patterns across different energies and charges.

Findings

No anisotropies detected at energies above E/Z for Z=6, 13, 26

Constraints on proton fractions at lower energies based on anisotropy absence

Supports heavy nuclei dominance in ultra-high energy cosmic rays

Abstract

The Pierre Auger Collaboration has reported evidence for anisotropy in the distribution of arrival directions of the cosmic rays with energies $E>E_{th}=5.5\times 10^{19}$ eV. These show a correlation with the distribution of nearby extragalactic objects, including an apparent excess around the direction of Centaurus A. If the particles responsible for these excesses at $E>E_{th}$ are heavy nuclei with charge $Z$, the proton component of the sources should lead to excesses in the same regions at energies $E/Z$. We here report the lack of anisotropies in these directions at energies above $E_{th}/Z$ (for illustrative values of $Z=6,\ 13,\ 26$). If the anisotropies above $E_{th}$ are due to nuclei with charge $Z$, and under reasonable assumptions about the acceleration process, these observations imply stringent constraints on the allowed proton fraction at the lower energies.