# An Original Mechanism for the Acceleration of Ultra-High-Energy Cosmic Rays

## TL;DR

This paper proposes a new one-shot acceleration mechanism called 'espresso' that boosts cosmic rays to ultra-high energies in relativistic jets, explaining observed compositions and source distributions.

## Contribution

It introduces the 'espresso' acceleration mechanism, showing how relativistic flows can efficiently re-accelerate cosmic rays to ultra-high energies, aligning with observational data.

## Key findings

- Re-acceleration by relativistic jets can reach 10^20 eV energies.
- The model explains the chemical composition of UHECRs.
- Potential sources include blazars like Mrk 421.

## Abstract

We suggest that ultra-high-energy (UHE) cosmic rays (CRs) may be accelerated in ultra-relativistic flows via a one-shot mechanism, the "espresso" acceleration, in which already-energetic particles are generally boosted by a factor of $\sim\Gamma^2$ in energy, where $\Gamma$ is the flow Lorentz factor. More precisely, we consider blazar-like jets with $\Gamma\gtrsim 30$ propagating into a halo of "seed" CRs produced in supernova remnants, which can accelerate UHECRs up to $10^{20}$\,eV. Such a re-acceleration process naturally accounts for the chemical composition measured by the Pierre Auger Collaboration, which resembles the one around and above the knee in the CR spectrum, and is consistent with the distribution of potential sources in the local universe, particularly intriguing is the coincidence of the powerful blazar Mrk 421 with the hotspot reported by the Telescope Array Collaboration.