Kinetic Beaming of Emission Produced by Radiatively Cooled Relativistic Magnetic Reconnection

Relativistic collisionless magnetic reconnection is often invoked to explain high-energy emission observed in astrophysical sources such as pulsar wind nebulae, gamma-ray bursts, and relativistic blazar jets. Reconnection produces nonthermal spectra, as are commonly observed in these sources. Reconnection may also explain distinctive short-timescale flaring. Such flares could arise because reconnection-accelerated particles—and their emitted photons—tend to be focused into beams with greater collimation at higher energies. This “kinetic beaming” may produce rapid high-energy flares when beams sweep across the observer’s line of sight. Using large-scale 2D particle-in-cell simulations, we systematically investigate the robustness (i.e. existence and duration) of kinetic beaming in emission powered by reconnection subject to external inverse Compton (IC) cooling. We find that only strongly cooled energetic particle beams can radiate most of their energy before they disperse. Thus strong cooling promotes kinetic beaming, while weak cooling suppresses it. Our results support the view that reconnection may power rapid IC gamma-ray flares in sources such as blazar jets.