Beaming of particles and synchrotron radiation in relativistic magnetic reconnection

Relativistic reconnection has been invoked as a mechanism for particle acceleration in numerous astrophysical systems. According to idealized analytical models, reconnection produces a bulk relativistic outflow emerging from the reconnection sites (X-points). The resulting radiation is therefore highly beamed. Using two-dimensional particle-in-cell simulations, we investigate particle and radiation beaming, finding a very different picture. Instead of having a relativistic average bulk motion with an isotropic electron velocity distribution in its rest frame, we find that the bulk motion of the particles in X-points is similar to their Lorentz factor γ, and the particles are beamed within $\sim 5/\gamma$. On the way from the X-point to the magnetic islands, particles turn in the magnetic field, forming a fan confined to the current sheet. Once they reach the islands they isotropize after completing a full Larmor gyration and their radiation is no longer strongly beamed. The radiation pattern at a given frequency depends on where the corresponding emitting electrons radiate their energy. Lower-energy particles that cool slowly spend most of their time in the islands and their radiation is not highly beamed. Only particles that quickly cool at the edge of the X-points generate a highly beamed fan-like radiation pattern. The radiation emerging from these fast cooling particles is above the burn-off limit (∼100 MeV in the overall rest frame of the reconnecting plasma). This has significant implications for models of gamma-ray bursts and active galactic nuclei that invoke beaming in that frame at much lower energies.