Ellis R Owen (NTHU)
Ultra-high-energy cosmic rays of energies 10^18-10^20 eV, accelerated in violent astrophysical environments, can undergo interactions with cosmic microwave background radiation via photo-hadronic processes, leading to strong attenuation and a restriction to their propagation distance of several 10s of Mpc. This sets the traditional boundary of the Greisen-Zatsepin-Kuz'min (GZK) horizon, within which the sources that generate ultra-energetic cosmic rays detected on Earth would be located. In this talk, I will show that the conventional GZK horizon can be extended to form a set of multi-particle CR horizons when photo-spallation of the heavy nuclear CRs is taken into account. A significant consequence of this is the emergence of an isotropic background component in the observed flux of ultra high-energy cosmic rays, coexisting with the anisotropic foreground component that are associated with nearby cosmic accelerators. Multi-particle cosmic ray horizons, which evolve over redshift, are determined by the cosmic ray nuclear composition and are therefore dependent on the source populations and source evolutionary histories.