"Espresso" Acceleration of Ultra-high-energy Cosmic Rays

Abstract

We propose that ultra-high-energy (UHE) cosmic rays (CRs) above $10^{18}$eV are produced in relativistic jets of powerful active galactic nuclei via an original mechanism, which we dub "espresso" acceleration: "seed" galactic CRs with energies $\lesssim 10^{17}$eV that penetrate the jet sideways receive a "one-shot'" boost of a factor of $\sim\Gamma^2$ in energy, where $\Gamma$ is the Lorentz factor of the relativistic flow. For typical jet parameters, a few percent of the CRs in the host galaxy can undergo this process, and powerful blazars with $\Gamma\gtrsim 30$ may accelerate UHECRs up to more than $10^{20}$eV. The chemical composition of espresso-accelerated UHECRs is determined by that at the Galactic CR knee and is expected to be proton-dominated at $10^{18}$eV and increasingly heavy at higher energies, in agreement with recent observations made at the Pierre Auger Observatory.