Simulations of ultra-high Energy Cosmic Rays in the local Universe and the origin of Cosmic Magnetic Fields

TL;DR

This study simulates ultra-high energy cosmic ray propagation in the local Universe considering various magnetic field origins, revealing that source distribution influences anisotropy at the highest energies, and matching observed dipole signals with heavy nuclei models.

Contribution

It introduces detailed simulations of cosmic ray propagation incorporating different magneto-genesis scenarios and source distributions, providing insights into anisotropy and composition effects.

Findings

Magneto-genesis scenarios have limited impact on anisotropy.

High-energy anisotropy is mainly due to nearby source distribution.

Observed dipole can be explained by heavy nuclei clustering.

Abstract

We simulate the propagation of cosmic rays at ultra-high energies, $\gtrsim 10^{18}$ eV, in models of extragalactic magnetic fields in constrained simulations of the local Universe. We use constrained initial conditions with the cosmological magnetohydrodynamics code {\sc ENZO}. The resulting models of the distribution of magnetic fields in the local Universe are used in the \crpropa code to simulate the propagation of ultra-high energy cosmic rays. We investigate the impact of six different magneto-genesis scenarios, both primordial and astrophysical, on the propagation of cosmic rays over cosmological distances. Moreover, we study the influence of different source distributions around the Milky Way. Our study shows that different scenarios of magneto-genesis do not have a large impact on the anisotropy measurements of ultra-high energy cosmic rays. However, at high energies above the GZK-limit, there is anisotropy caused by the distribution of nearby sources, independent of the magnetic field model. This provides a chance to identify cosmic ray sources with future full-sky measurements and high number statistics at the highest energies. Finally, we compare our results to the dipole signal measured by the Pierre Auger Observatory. All our source models and magnetic field models could reproduce the observed dipole amplitude with a pure iron injection composition. Our results indicate that the dipole is observed due to clustering of secondary nuclei in direction of nearby sources of heavy nuclei. A light injection composition is disfavoured by the non-observation of anisotropy at energies of $4-8 \rm\ EeV$.