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

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 $\mathtt{\text{UMAREL}}$, 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 $E\lesssim 3\times {10}^{19}\mathrm{e}\mathrm{V}$ through an effective Magnetic Horizon (MH). We estimate $R_{\mathrm{M}\mathrm{H}}\sim 50\mathrm{M}\mathrm{p}\mathrm{c}$ for protons with $E={10}^{18}\mathrm{e}\mathrm{V}$ and $R_{\mathrm{M}\mathrm{H}}\sim 150\mathrm{M}\mathrm{p}\mathrm{c}$ for protons with $E={10}^{19}\mathrm{e}\mathrm{V}$.} {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.}