Cosmic-Ray Anisotropy from Large Scale Structure and the effect of magnetic horizons

TL;DR

This study investigates how large scale structure and magnetic fields influence the anisotropy of ultra-high energy cosmic rays, revealing a significant LSS-induced dipole amplitude affected by magnetic horizons and fields.

Contribution

It provides a detailed analysis of the impact of intergalactic and galactic magnetic fields on UHECR anisotropy using constrained local Universe simulations.

Findings

LSS induces a ~10% dipole anisotropy at 11.5 EeV.

The UHECR horizon is significantly affected by the IGMF strength.

Galactic magnetic fields modulate the observed anisotropy.

Abstract

Motivated by the ~7 % 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, 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 ($\lesssim350$ 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 $A_1\sim10\%$, 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 (GMF) and we discuss its effect on the LSS-induced anisotropy.