Propagation of Ultra High Energy Cosmic Rays in Extragalactic Magnetic Fields: A view from cosmological simulations

TL;DR

This study uses cosmological simulations to analyze how extragalactic magnetic fields influence the propagation, spectra, and anisotropy of ultra high energy cosmic rays, providing constraints on primordial magnetic field strengths.

Contribution

It introduces a comprehensive simulation framework combining CRPropa and ENZO to assess the impact of different magnetic field origins and strengths on cosmic ray propagation and anisotropy.

Findings

Arrival spectra are insensitive to magnetic field distribution.

Source clustering affects cosmic ray anisotropy.

Void magnetic fields must be around 0.1 nG to match observed isotropy.

Abstract

We use the CRPropa code to simulate the propagation of ultra high energy cosmic rays (with energy $\geq 10^{18} \rm eV$ and pure proton composition) through extragalactic magnetic fields that have been simulated with the cosmological ENZO code.We test both primordial and astrophysical magnetogenesis scenarios in order to investigate the impact of different magnetic field strengths in clusters, filaments and voids on the deflection of cosmic rays propagating across cosmological distances. We also study the effect of different source distributions of cosmic rays around simulated Milky-Way like observers. Our analysis shows that the arrival spectra and anisotropy of events are rather insensitive to the distribution of extragalactic magnetic fields, while they are more affected by the clustering of sources within a $\sim 50$ Mpc distance to observers. Finally, we find that in order to reproduce the observed degree of isotropy of cosmic rays at $\sim $ EeV energies, the average magnetic fields in cosmic voids must be $\sim 0.1 \rm \ nG$, providing limits on the strength of primordial seed fields.