Probing cosmic-ray anisotropy at ultra-high energy

Abstract

Although known for decades, the origin of ultra-high-energy cosmic rays is still baffling scientists. Recent data from the Pierre Auger Observatory and the Telescope Array have provided compelling evidence of anisotropy in their arrival directions, indicating an extragalactic origin, but the sources are yet to be identified. This paper aims to test against the observed anisotropy of two different models of extragalactic candidate sources, namely, starburst galaxies (SBGs) and active galactic nuclei (AGNs), by studying in depth the propagation of ultra-high-energy cosmic rays in the Galactic and extragalactic environments. The calculations are carried out using the software framework CRPropa 3, taking into account all relevant particle interactions and magnetic deflections. One-dimensional as well as three-dimensional simulations are considered, the former to study the energy spectra of particles reaching the Earth and the latter to address anisotropy. It is shown that nearby SBGs, and to a less extent nearby AGNs, can explain some of the observed medium-scale anisotropies ($\sim$10$^\circ$–30$^\circ$), but no known nearby SBG or AGN can provide a significant contribution to the strong large-scale anisotropy (${\sim } 45^\circ$), recently reported by the Pierre Auger Observatory.