Exploring the High-Energy Cosmic Ray Spectrum with a Toy Model of Cosmic Ray Diffusion

Abstract

We introduce a static toy model of the cosmic ray (CR) universe in which cosmic ray propagation is taken to be diffusive and cosmic ray sources are distributed randomly with a density the same as that of local L* galaxies, $5 \times 10^{-3}$ Mpc$^{-3}$. These sources "fire" at random times through the history of the universe but with a set expectation time for the period between bursts. Our toy model model captures much of the essential CR physics despite its simplicity and, moreover, broadly reproduces CR phenomenology for reasonable parameter values and without extreme fine-tuning. Using this model we investigate -- and find tenable -- the idea that the Milky Way may itself be a typical high-energy cosmic ray source. We also consider the possible phenomenological implications of the magnetic CR horizon for the overall cosmic ray spectrum observed at Earth. Finally, we show that anisotropy studies should most profitably focus on cosmic rays detected at energies above the so-called GZK cut-off, $\sim 6 \times 10^{19}$ eV.