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We report on the first study of the angular distribution of energeticparticles and radiation generated in relativistic collisionlesselectron-positron pair plasma reconnection, using two-dimensionalparticle-in-cell simulations. We discover a strong anisotropy of the particlesaccelerated by reconnection and the associated strong beaming of theirradiation. The focusing of particles and radiation increases with their energy;in this sense, this "kinetic beaming" effect differs fundamentally from therelativistic Doppler beaming usually invoked in high-energy astrophysics, inwhich all photons are focused and boosted achromatically. We also present, forthe first time, the modeling of the synchrotron emission as seen by an externalobserver during the reconnection process. The expected lightcurves compriseseveral bright symmetric sub-flares emitted by the energetic beam of particlessweeping across the line of sight intermittently, and exhibit super-fast timevariability as short as about one tenth of the system light-crossing time. Theconcentration of the energetic particles into compact regions inside magneticislands and particle anisotropy explain the rapid variability. This radiativesignature of reconnection can account for the brightness and variability of thegamma-ray flares in the Crab Nebula and in blazars.

Beaming and rapid variability of high-energy radiation from relativistic pair plasma reconnection