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

TL;DR

This paper demonstrates through kinetic simulations that magnetically dominated turbulence can rapidly accelerate ultra-high-energy cosmic rays, producing a characteristic energy spectrum that aligns with observational data from the Pierre Auger Observatory.

Contribution

The study provides the first first-principles simulation evidence that magnetically dominated turbulence accelerates UHECRs with a specific energy cutoff and spectral shape, matching observational data.

Findings

Particles are accelerated rapidly, forming a power-law spectrum with a sharp cutoff.

Energy-dependent escape times follow a E^{-1/3} relation.

The simulated spectrum fits Pierre Auger data with a spectral index of about 2.1.

Abstract

Ultra-High-Energy Cosmic Rays (UHECRs), particles characterized by energies exceeding $10^{18}$ 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 $f_{\rm cut}\left({E, E_{\rm cut}}\right) = {\text{sech}}\left[ ( {{E}/{E_{\rm cut}}} )^2 \right]$. Particle escape from the turbulent accelerating region is energy-dependent, with $t_{\rm esc} \propto E^{-\delta}$ and $\delta \sim 1/3$. The resulting particle flux from the accelerator follows $dN/dEdt \propto E^{-s} {\text{sech}}\left[ ( {{E}/{E_{\rm cut}}} )^2 \right]$, with $s \sim 2.1$. 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 $s = 2.1^{+0.06}_{-0.13}$. Our first-principles results indicate that particle acceleration by magnetically dominated turbulence may constitute the physical mechanism responsible for UHECR acceleration.