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

TL;DR

This paper investigates how relativistic jets in active galactic nuclei can produce ultra-high-energy cosmic rays through a process called espresso acceleration, with the addition of magnetic turbulence effects influencing lower-energy cosmic rays.

Contribution

It extends previous simulations by including sub-grid magnetic turbulence modeling, revealing its impact on cosmic ray acceleration and spectrum shaping.

Findings

Stochastic acceleration enhances lower-energy cosmic rays.

Highest-energy cosmic rays are produced by espresso acceleration regardless of turbulence.

Spectral slopes and composition align with observed UHECR phenomenology.

Abstract

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. Yet, the particles that achieve the highest energies (up to the Hillas limit) are invariably produced by espresso acceleration and their spectrum is independent of the assumed sub-grid scattering rate.