# Implications of Strong Intergalactic Magnetic Fields for Ultra-High-Energy Cosmic-Ray Astronomy

## TL;DR

This study explores how strong intergalactic magnetic fields influence ultra-high-energy cosmic-ray propagation, suggesting potential for cosmic-ray astronomy despite magnetic deflections, with results sensitive to primordial magnetic conditions.

## Contribution

It provides the first detailed analysis of UHECR propagation in magnetised cosmic web simulations considering different primordial magnetic spectra.

## Key findings

- Magnetic fields in voids can alter the cosmic-ray spectrum and composition significantly.
- Deflections of 50 EeV protons are less than 15° in a substantial sky fraction.
- UHECR astronomy remains feasible depending on magnetic field strength and source distribution.

## Abstract

We study the propagation of ultra-high-energy cosmic rays in the magnetised cosmic web. We focus on the particular case of highly magnetised voids ($B \sim \text{nG}$), using the upper bounds from the Planck satellite. The cosmic web was obtained from purely magnetohydrodynamical cosmological simulations of structure formation considering different power spectra for the seed magnetic field in order to account for theoretical uncertainties. We investigate the impact of these uncertainties on the propagation of cosmic rays, showing that they can affect the measured spectrum and composition by up to $\simeq 80\%$ and $\simeq 5\%$, respectivelly. In our scenarios, even if magnetic fields in voids are strong, deflections of 50 EeV protons from sources closer than $\sim\;$50 Mpc are less than $15^\circ$ in approximately 10-50% of the sky, depending on the distribution of sources and magnetic power spectrum. Therefore, UHECR astronomy might be possible in a significant portion of the sky depending on the primordial magnetic power spectrum, provided that protons constitute a sizeable fraction of the observed UHECR flux.