Particle Acceleration in Magnetically Dominated Plasma Turbulence

Nature's most powerful high-energy sources are capable of accelerating particles to high energy 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 first-principles fully kinetic simulations, we show that strong turbulence in magnetically dominated plasmas generates a nonthermal particle spectrum with a hard power-law range within a few eddy turnover times. Low pitch-angle electrons can significantly exceed the nominal radiation-reaction limit, before abruptly cooling down. The electron energy spectrum becomes harder over time owing to particle cooling with an energy-dependent pitch-angle anisotropy. The resulting synchrotron spectrum is hard, with a significant fraction of radiative power emitted above the nominal radiation reaction limit. 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.