Espresso and Stochastic Acceleration of Ultra-high-energy Cosmic Rays in Relativistic Jets

Understand

In the espresso scenario, ultra-high-energy (UHE) cosmic rays (CRs) are produced via a one-shot reacceleration of galactic-like CRs in the relativistic jets of active galactic nuclei, independently of the scattering rate dictated by magnetic fluctuations.

• In Mbarek & Caprioli (2019), we traced test-particle CRs in high-resolution magnetohyrodynamic (MHD) jet simulations and found that the associated spectral slope, chemical composition, and anisotropy are consistent with UHECR phenomenology.
• In this work, we extend such an analysis by including sub-grid pitch-angle scattering to model small-scale magnetic turbulence that cannot be resolved by MHD simulations.
• We find that a large scattering rate unlocks stochastic acceleration and fosters the energization of lower-energy CRs, which eventually leads to harder UHECR spectra.