Acceleration of Ultra-high-energy Cosmic Rays in Relativistic Jets of Giant Radio Galaxies

Abstract

Acceleration of Ultra-high-energy Cosmic Rays in Relativistic Jets of Giant Radio Galaxies

Hyesung Kang^1, Dongsu Ryu^2, Jeongbhin Seo^3

1Depart. of Earth Sciences, Pusan National University, Rep. of Korea 2Depart. of Physics, College of Natural Sciences, UNIST, Rep. of Korea 3Los Alamos National Laboratory, Los Alamos, United States

In this study, we investigate cosmic ray (CR) acceleration processes within FR-I and FR-II radio galaxies through a series of relativistic hydrodynamic simulations, encompassing relativistic jets characterized by varying powers and bulk Lorentz factors. Monte Carlo simulations are subsequently employed to model the transport, scattering, and energy changes of CR particles, facilitating the calculation of particle acceleration within the simulated jet-induced flows. Our results indicate that diffusive shock acceleration in the jet-spine and backflow energizes CRs up to approximately 1 EeV. At higher energies, however, relativistic shear acceleration across the jet-backflow interface becomes more significant. The time-asymptotic energy spectrum of CR particles escaping from the jet system can be represented by a double power law with an extended exponential cutoff. We demonstrate that the charge-dependent break energy is primarily governed by the jet power, but with weak dependencies on the jet size and background density. Notably, the conventional exponential cutoff is extended through the energy boost achieved via non-gradual shear acceleration, proportional to the square of the mean Lorentz factor of the jet. Our analysis demonstrates that more relativistic jets with higher bulk Lorentz factors can generate energy spectra extending to higher energies, resulting in elevated particle flux and a lighter mass composition at the high-energy tail. These findings provide valuable insights into the acceleration of ultra-high-energy cosmic rays within giant radio galaxies.