Contribution of Local Radio Galaxies to Ultra-High-Energy Cosmic Rays Observed at Earth

Abstract

We investigate the potential contribution of local radio galaxies (RGs) to ultra-high- energy cosmic rays (UHECRs) observed at Earth. Our previous studies, which combined relativistic hydrodynamic simulations with Monte Carlo particle transport, demonstrated that UHECRs can be accelerated to energies exceeding 1020 eV through mechanisms such as shocks, turbulence, and relativistic shear in jet-driven flows. These studies showed that the time-asymptotic energy spectrum of escaping UHECRs from Fanaroff-Riley (FR) type RGs follows a double power law with an extended exponential cutoff, primarily shaped by relativistic shear acceleration. In this work, we apply this newly derived source spectrum to prominent RGs and use the publicly available CRPropa code to model the propagation of UHECRs from Virgo A, Centaurus A, Fornax A, and Cygnus A. We analyze the resulting energy spectrum and mass composition of UHECRs arriving at Earth. Our results indicate that Virgo A, with its higher Lorentz factor, produces a higher flux and a lighter mass composition than Centaurus A and Fornax A, which have lower Lorentz factors. Cygnus A, despite being a powerful FR-II galaxy, contributes minimally due to its large distance. We discuss the implications of these findings for potential observational differences between the Telescope Array in the Northern Hemisphere and the Pierre Auger Observatory in the Southern Hemisphere.