Exploring the Origins of Ultra-high-energy Cosmic Rays: Insights from Simulated Relativistic Jets

Abstract

In this study, we investigate the possibility that relativistic jets from active galactic nuclei (AGNs) could be a significant source of ultra-high-energy cosmic rays (UHECRs). To do so, we perform a set of relativistic hydrodynamic simulations to model radio jets with a range of jet powers and bulk Lorentz factors. We then calculate the acceleration of particles in the simulated jet-induced flows through various mechanisms, including diffusive shock acceleration, turbulent acceleration, and shear acceleration. We find that in high-power, collimated jets such as those found in FR-II radio galaxies, while gradual shear acceleration dominates for the energization of UHECRs above ~EeV, non-gradual shear acceleration is the primary process for the production of the highest energy UHECRs. In lower-power, decollimated jets such as those found in FR-I types, gradual shear acceleration is most important due to the dispersion of the shear flow. We observe that the energy spectrum of the particles escaping from the simulated jets can be represented by a double power law with an exponential cutoff. The shape of the spectrum is determined by the escape from the turbulent backflow as well as the boosting due to non-gradual shear acceleration. These findings provide insight into the acceleration mechanisms of particles in relativistic jets and shed light on the origin of UHECRs.