2015ApJ...811L..38C

We propose that ultra-high-energy (UHE) cosmic rays (CRs) above 10¹⁸ 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 ≲10¹⁷ eV that penetrate the jet sideways receive a ''one-shot'' boost of a factor of ~Γ² in energy, where Γ 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 Γ ≳ 30 may accelerate UHECRs up to more than 10²⁰ 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 10¹⁸ eV and increasingly heavy at higher energies, in agreement with recent observations made at the Pierre Auger Observatory.