Model Spectrum of Ultra-High-Energy Cosmic Rays Accelerated in FR-I Radio Galaxy Jets

TL;DR

This study models how relativistic jets in FR-I radio galaxies accelerate cosmic rays to ultra-high energies, revealing a double power-law spectrum shaped by shock and shear processes, with implications for UHECR origins.

Contribution

It provides a detailed simulation-based model of cosmic ray acceleration in FR-I jets, highlighting the role of jet dynamics and shear acceleration in shaping the energy spectrum.

Findings

CRs gain energy via shock and shear acceleration

The spectrum shows a double power-law with a break at E_break

The cutoff energy depends on confinement and escape processes

Abstract

Nearby radio galaxies (RGs) of Fanaroff-Riley Class I (FR-I) are considered possible sites for the production of observed ultra-high-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, $\langle\Gamma\rangle_{\rm{spine}}$. When $\langle\Gamma\rangle_{\rm{spine}}\gtrsim$ several, the jet spine remains almost unimpeded, but for $\langle\Gamma\rangle_{\rm{spine}}\lesssim$ a few, substantial jet deceleration occurs. CRs gain energy mainly through diffusive shock acceleration for $E\lesssim1$~EeV and shear acceleration for $E\gtrsim1$~EeV. The time-asymptotic energy spectrum of CRs escaping from the jet can be modeled by a double power law, transitioning from $\sim E^{-0.6}$ to $\sim E^{-2.6}$ around a break energy, $E_{\rm{break}}$, with an exponential cutoff at $E_{\rm{break}}\langle\Gamma\rangle_{\rm{spine}}^2$. $E_{\rm{break}}$ 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 accelerations, and the confinement-escape processes within the cocoon. The exponential cutoff is determined by non-gradual shear acceleration that boosts the energy of high-energy CRs by a factor of $\sim \langle\Gamma\rangle_{\rm{spine}}^2$. 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.