Anisotropy signal of UHECRs from a structured magnetized Universe

TL;DR

This study simulates ultra-high-energy cosmic ray propagation in a magnetized universe to analyze how source distribution and magnetic fields influence observed anisotropy, revealing challenges in matching observed dipole and quadrupole signals.

Contribution

It introduces detailed 3D simulations of UHECR propagation through structured magnetic fields considering various source scenarios, highlighting the dominant role of source distribution in anisotropy.

Findings

Source distribution significantly impacts anisotropy patterns.

Magnetic field models have a lesser effect compared to source distribution.

Predicted quadrupole strength exceeds observational limits.

Abstract

The surprising isotropy of the ultra-high-energy cosmic ray (UHECR) sky makes it difficult to identify their sources. Observables such as energy spectrum, mass composition and arrival directions are affected by interactions with background photon fields and by deflection in the extragalactic and galactic magnetic fields (EGMF and GMF). In this work, we simulate the propagation of UHECRs with energy above 8 EeV in magnetized replicas of the local Universe, obtained from constrained simulations of the Large Scale Structure. We obtain the real magnetic deflection in structured EGMF models with realistic three-dimensional simulations. We investigate different scenarios for the UHECR source distributions and densities. The effect of the GMF can be different depending on the field model considered. In this work we consider the JF12 model by mapping the arrival directions at the edge of the galaxy to those at Earth. We study the arrival direction distribution of the propagated UHECRs, and in particular their angular power spectrum, dipole and quadrupole moments. We find that the properties of the source distribution affect the cosmic ray anisotropy more than the EGMF model considered. In particular, the low multipole components depend on both the source distribution and the density. We also find that it is difficult to simultaneously reproduce the observed dipole and quadrupole values above EeV. In general, we predict too large a quadrupole strength, incompatible with observations.