Ultra-High-Energy Cosmic Rays Accelerated by Magnetically Dominated Turbulence

Abstract

Ultra-high-energy cosmic rays (UHECRs), particles characterized by energies exceeding 1018 eV, are generally believed to be accelerated electromagnetically in high-energy astrophysical sources. One promising mechanism of UHECR acceleration is magnetized turbulence. We demonstrate from first principles, using fully kinetic particle-in-cell simulations, that magnetically dominated turbulence accelerates particles on a short timescale, producing a power-law energy distribution with a rigidity-dependent, sharply defined cutoff well approximated by the form . Particle escape from the turbulent accelerating region is energy dependent, with tesc ∝ E−δ and δ ∼ 1/3. The resulting particle flux from the accelerator follows , with s ∼ 2.1. We fit the Pierre Auger Observatory’s spectrum and composition measurements, taking into account particle interactions between acceleration and detection, and show that the turbulence-associated energy cutoff is well supported by the data, with the best-fitting spectral index being . Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration.