Origin of Ultrahigh Energy Galactic Cosmic Rays: The Isotropy Problem

TL;DR

This paper investigates the origin of ultrahigh-energy galactic cosmic rays below 4 EeV, highlighting the challenges in explaining their observed isotropy and composition, and proposing constraints on source properties and propagation models.

Contribution

It introduces a Monte-Carlo diffusion model considering source intermittency and derives new constraints on UHECR sources based on anisotropy and composition observations.

Findings

Reproducing observed anisotropy and composition is challenging under standard assumptions.

Long escape times or heavier composition are required to match observations.

The extragalactic component likely dominates below the ankle at 4 EeV.

Abstract

We study the propagation of ultra-high-energy cosmic rays (UHECR) in the Galaxy, concentrating on the energy range below the ankle in the spectrum at 4 EeV. A Monte-Carlo method, based on analytical solutions to the time-dependent diffusion problem, is used to account for intermittency by placing sources at random locations. Assuming a source population that scales with baryon mass density or star formation (e.g. long GRB), we derive constraints arising from intermittency and the observational limits on the composition and anisotropy. It is shown that the composition and anisotropy at 1e18 eV are difficult to reproduce and require that either a) the particle mean free path is much smaller than a gyroradius, implying the escape time is very long, b) the composition is heavier than suggested by recent Auger data, c) the ultrahigh-energy sub-ankle component is mostly extragalactic, or d) we are living in a rare lull in the ultrahigh-energy cosmic-ray production, and the current ultrahigh-energy cosmic-ray intensity is far below the Galactic time average. We therefore recommend a strong observational focus on determining the UHECR composition around 1e18 eV.