Ultrahigh-energy dipole and beyond

TL;DR

This paper investigates the origin and energy dependence of the large-scale dipole in ultra-high energy cosmic rays, using analytical and semi-analytical models to understand the influence of sources and magnetic fields.

Contribution

It introduces a novel analytical framework for modeling the dipole distribution and its evolution with energy, considering source characteristics and magnetic field effects.

Findings

Local sources can cause a strong dipole growth with energy.

A transition from dipolar to non-dipolar regimes at high energies is possible.

Implications for source density, magnetic fields, and composition are discussed.

Abstract

Recent data from the Pierre Auger Observatory has revealed the presence of a large-scale dipole in the arrival direction distribution of ultra-high energy cosmic rays (UHECR). In this work, we build up an understanding of the diffusive origin of such a dipolar behavior as well as its dependency on energy and astrophysical source assumptions such as extragalactic magnetic field strength and cosmic ray composition. We present a novel analytical approach for calculating the angular distribution of CR coming from a single source and discuss the regimes in which the steady-state dipole result is expected. We also present a semianalytical method for calculating the evolution with energy of the resultant dipole for an ensemble of sources. We show that a local source allows for a strong growth of the dipole with energy over a large energy range. The possibility of a transition from a dipolar to non-dipolar regime at the highest energies and its implications for the source density, magnetic field intensity, and cosmic ray composition are discussed.