The Impact of the Magnetised Cosmic Web on Ultra High Energy Cosmic Ray Propagation

TL;DR

This study uses cosmological simulations and a new numerical framework to quantify how extragalactic magnetic fields influence the propagation and observed flux of ultra-high-energy cosmic rays, highlighting the importance of the magnetic horizon effect.

Contribution

It introduces UMAREL, a parallel code that models UHECR propagation through evolving cosmic web magnetic fields, accounting for the magnetic horizon effect.

Findings

Magnetic fields suppress UHECR flux below 3×10^{19} eV.

Estimated magnetic horizon radius is ~50 Mpc at 10^{18} eV.

Estimated magnetic horizon radius is ~150 Mpc at 10^{19} eV.

Abstract

The origin of ultra-high-energy cosmic rays (UHECRs) remains an open question. Extragalactic magnetic fields can modify their propagation and, at sufficiently low energies, suppress the observed flux through the magnetic horizon (MH) effect.} {We quantify the impact of the MH on the propagation of UHECR protons using cosmological simulations and a dedicated numerical framework that follows cosmic rays in a time-evolving background.} {We use \texttt{UMAREL}, a parallel code developed for this study, to propagate UHECR protons through a cosmological volume simulated with ENZO. The magnetic-field configurations are chosen to be consistent with recent radio constraints on magnetic fields in cosmic-web filaments. Unlike stationary approaches, we follow particle trajectories through a sequence of time-evolving snapshots and compare the resulting arrival properties with those in an unmagnetised reference model.} {We find that observationally motivated extragalactic magnetic fields progressively suppress the flux of arriving protons below \(E \lesssim 3 \times 10^{19}\,\mathrm{eV}\) through an effective Magnetic Horizon (MH). We estimate \(R_{\mathrm{MH}} \sim 50\,\mathrm{Mpc}\) for protons with \(E = 10^{18}\,\mathrm{eV}\) and \(R_{\mathrm{MH}} \sim 150\,\mathrm{Mpc}\) for protons with \(E = 10^{19}\,\mathrm{eV}\).} {The MH generated by extragalactic magnetic fields must be taken into account when modelling UHECR propagation and interpreting the spectrum observed in the local Universe.}