  We report on the first study of the angular distribution of energetic
particles and radiation generated in relativistic collisionless
electron-positron pair plasma reconnection, using two-dimensional
particle-in-cell simulations. We discover a strong anisotropy of the particles
accelerated by reconnection and the associated strong beaming of their
radiation. The focusing of particles and radiation increases with their energy;
in this sense, this "kinetic beaming" effect differs fundamentally from the
relativistic Doppler beaming usually invoked in high-energy astrophysics, in
which all photons are focused and boosted achromatically. We also present, for
the first time, the modeling of the synchrotron emission as seen by an external
observer during the reconnection process. The expected lightcurves comprise
several bright symmetric sub-flares emitted by the energetic beam of particles
sweeping across the line of sight intermittently, and exhibit super-fast time
variability as short as about one tenth of the system light-crossing time. The
concentration of the energetic particles into compact regions inside magnetic
islands and particle anisotropy explain the rapid variability. This radiative
signature of reconnection can account for the brightness and variability of the
gamma-ray flares in the Crab Nebula and in blazars.
