Constraints on the source of ultra-high energy cosmic rays using anisotropy vs chemical composition

TL;DR

This paper investigates how anisotropy signals and chemical composition of ultra-high energy cosmic rays can constrain their sources, focusing on secondary proton production from heavy nuclei and implications for source distance and metallicity.

Contribution

It introduces a method to relate anisotropy signals to source distance and metallicity constraints based on chemical composition and secondary proton production.

Findings

Sources responsible for observed anisotropies are within 20-200 Mpc depending on composition.

High metallicity in sources is required to suppress secondary proton anisotropies.

Constraints can help identify or exclude potential cosmic ray sources.

Abstract

The joint analysis of anisotropy signals and chemical composition of ultra-high energy cosmic rays offers strong potential for shedding light on the sources of these particles. Following up on an earlier idea, this paper studies the anisotropies produced by protons of energy >E/Z, assuming that anisotropies at energy >E have been produced by nuclei of charge Z, which share the same magnetic rigidity. We calculate the number of secondary protons produced through photodisintegration of the primary heavy nuclei. Making the extreme assumption that the source does not inject any proton, we find that the source(s) responsible for anisotropies such as reported by the Pierre Auger Observatory should lie closer than ~20-30, 80-100 and 180-200 Mpc if the anisotropy signal is mainly composed of oxygen, silicon and iron nuclei respectively. A violation of this constraint would otherwise result in the secondary protons forming a more significant anisotropy signal at lower energies. Even if the source were located closer than this distance, it would require an extraordinary metallicity >120, 1600, 1100 times solar metallicity in the acceleration zone of the source, for oxygen, silicon and iron respectively, to ensure that the concomitantly injected protons not to produce a more significant low energy anisotropy. This offers interesting prospects for constraining the nature and the source of ultra-high energy cosmic rays with the increase in statistics expected from next generation detectors.