The extragalactic ultra-high energy cosmic-ray dipole

TL;DR

This paper investigates whether the observed dipole anisotropy in ultra-high energy cosmic rays can be explained by large-scale structure and magnetic fields, finding that nearby sources and certain magnetic field strengths are consistent with observations.

Contribution

It models the UHECR dipole anisotropy considering large-scale structure, intergalactic magnetic fields, and cosmic ray composition, providing new insights into the origin of the anisotropy.

Findings

Flux-weighted RMS dipole amplitude is ~8% before entering the Galaxy.

Dipole amplitude in 4-8 EeV range is ~5%.

Required IGMF strength is 5-30 nG, with sources within 300 Mpc.

Abstract

We explore the possibility that the recently detected dipole anisotropy in the arrival directions of~$>8$~EeV ultra-high energy cosmic-rays (UHECRs) arises due to the large-scale structure (LSS). We assume that the cosmic ray sources follow the matter distribution and calculate the flux-weighted UHECRs' RMS dipole amplitude taking into account the diffusive transport in the intergalactic magnetic field (IGMF). We find that the flux-weighted RMS dipole amplitude is $\sim8$% before entering the Galaxy. The amplitude in the [4-8] EeV is only slightly lower $\sim 5$%. The required IGMF is of the order of {5-30 nG}, and the UHECR sources must be relatively nearby, within $\sim$300 Mpc. The absence of statistically significant signal in the lower energy bin can be explained if the same nuclei specie dominates the composition in both energy bins and diffusion in the Galactic magnetic field (GMF) reduces the dipole of these lower rigidity particles. Photodisintegration of higher energy UHECRs could also reduce somewhat the lower energy dipole.