Pitch Angle Anisotropy Controls Particle Acceleration and Cooling in Radiative Relativistic Plasma Turbulence

TL;DR

This study uses radiative particle-in-cell simulations to reveal how pitch-angle anisotropy in magnetically dominated turbulence leads to efficient particle acceleration and hard synchrotron spectra, explaining high-energy astrophysical phenomena.

Contribution

It demonstrates that pitch-angle anisotropy in turbulent plasmas causes ultra-efficient particle acceleration and produces hard spectra, advancing understanding of high-energy astrophysical emissions.

Findings

Nonthermal particle spectrum with slope p ~ 1 forms rapidly.

Particles can exceed radiation-reaction limits before cooling.

Synchrotron spectrum is notably hard with s ~ 1.

Abstract

Nature's most powerful high-energy sources are capable of accelerating particles to high energy and radiate it away on extremely short timescales, even shorter than the light crossing time of the system. It is yet unclear what physical processes can produce such an efficient acceleration, despite the copious radiative losses. By means of radiative particle-in-cell simulations, we show that magnetically dominated turbulence in pair plasmas subject to strong synchrotron cooling generates a nonthermal particle spectrum with a hard power-law range (slope $p \sim 1$) within a few eddy turnover times. Low pitch-angle particles can significantly exceed the nominal radiation-reaction limit, before abruptly cooling down. The particle spectrum becomes even harder ($p < 1$) over time owing to particle cooling with an energy-dependent pitch-angle anisotropy. The resulting synchrotron spectrum is hard ($\nu F_\nu \propto \nu^s$ with $s \sim 1$). Our findings have important implications for understanding the nonthermal emission from high-energy astrophysical sources, most notably the prompt phase of gamma-ray bursts and gamma-ray flares from the Crab nebula.