Cosmic ray anisotropy from large-scale structure and the effect of magnetic horizons

Motivated by the ∼7 per cent dipole anisotropy in the distribution of ultra-high-energy cosmic rays (UHECRs) above 8 EeV, we explore the anisotropy induced by the large-scale structure (LSS), using constrained simulations of the local Universe and taking into account the effect of magnetic fields. The value of the intergalactic magnetic field (IGMF) is critical as it determines the UHECR cosmic horizon. We calculate the UHECR sky maps for different values of the IGMF variance and show the effect of the UHECR horizon on the observed anisotropy. The footprint of the local ( 350 Mpc) Universe on the UHECR background, a small angular scale enhancement in the Northern hemisphere, is seen. At 11.5 EeV (the median value of the energy bin at which the dipole has been reported), the LSS-induced dipole amplitude is A₁ ∼ 10 per cent, for IGMF in the range [0.3-3] nG for protons, helium and nitrogen, compatible with the rms value derived from the cosmic power spectrum. However at these energies, the UHECRs are also influenced by the Galactic magnetic field and we discuss its effect on the LSS-induced anisotropy.