Model Spectrum of Ultrahigh-energy Cosmic Rays Accelerated in FR-I Radio Galaxy Jets

Abstract

Nearby radio galaxies (RGs) of Fanaroff-Riley Class I (FR-I) are considered possible sites for the production of observed ultrahigh-energy cosmic rays (UHECRs). Among those, some exhibit blazar-like inner jets, while others display plume-like structures. We reproduce the flow dynamics of FR-I jets using relativistic hydrodynamic simulations. Subsequently, we track the transport and energization of cosmic ray (CR) particles within the simulated jet flows using Monte Carlo simulations. The key determinant of flow dynamics is the mean Lorentz factor of the jet-spine flow, Gamma spine. When Gamma spine greater than or similar to 6, the jet spine remains almost unimpeded, but for Gamma spine less than or similar to 3, substantial jet deceleration occurs. CRs gain energy mainly through diffusive shock acceleration for E less than or similar to 1 EeV and shear acceleration for E greater than or similar to 1 EeV. The time-asymptotic energy spectrum of CRs escaping from the jet can be modeled by a double power law, transitioning from similar to E -0.6 to similar to E -2.6 around a break energy, E break, with an exponential cutoff at Ebreak Gamma spine2 . E break is limited either by the Hillas confinement condition or by particle escape from the cocoon via fast spatial diffusion. The spectral slopes primarily arise from multiple episodes of shock and relativistic shear acceleration, and the confinement-escape processes within the cocoon. The exponential cutoff is determined by nongradual shear acceleration that boosts the energy of high-energy CRs by a factor of similar to Gamma spine2 . We suggest that the model spectrum derived in this work could be employed to investigate the contribution of RGs to the observed population of UHECRs.