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Many powerful and variable gamma-ray sources, including pulsar wind nebulae,active galactic nuclei and gamma-ray bursts, seem capable of acceleratingparticles to gamma-ray emitting energies efficiently over very short timescales. These are likely due to rapid dissipation of electromagnetic energy ina highly magnetized, relativistic plasma. In order to understand the genericfeatures of such processes, we have investigated simple models based onrelaxation of unstable force-free magnetostatic equilibria. In this work, wemake the connection between the corresponding plasma dynamics and the expectedradiation signal, using 2D particle-in-cell simulations that self-consistentlyinclude synchrotron radiation reaction. We focus on the lowest order unstableforce-free equilibrium in a 2D periodic box. We find that rapid variability,with modest apparent radiation efficiency as perceived by a fixed observer, canbe produced during the evolution of the instability. The "flares" areaccompanied by an increased polarization degree in the high energy band, withrapid variation in the polarization angle. Furthermore, the separation betweenthe acceleration sites and the synchrotron radiation sites for the highestenergy particles facilitates acceleration beyond the synchrotron radiationreaction limit. We also discuss the dynamical consequences of radiationreaction, and some astrophysical applications of this model. Our currentsimulations with numerically tractable parameters are not yet able to reproducethe most dramatic gamma-ray flares, e.g., from Crab Nebula. Highermagnetization studies are promising and will be carried out in the future.

Kinetic study of radiation-reaction-limited particle acceleration during the relaxation of unstable force-free equilibria