Reconnection-driven plasmoids in blazars: fast flares on a slow envelope
March 14, 2013
Abstract: Collimated, relativistic outflows (jets) have been extensively observed to originate from supermassive black holes in the centers of galaxies. When the jet flow is moving close to our line of sight (the so-called “blazar”), it results in an extremely bright, broadband and variable source of electromagnetic radiation. The mechanisms responsible for the energetic particles behind the jet emission remain poorly understood. The recent observations of powerful, minute-timescale flares from several blazars pose serious challenges to theoretical models for the blazar emission. The fast flaring requires extremely compact regions in the jet that boost their emission towards the observer at an extreme Doppler factor of delta~50 or larger. Furthermore, for TeV photons to avoid annihilation with ambient radiation at the source, the flares must originate far from the black hole (at multi light-year distance). In this talk, I will introduce the magnetic reconnection model for the blazar flaring. Focusing on the inherently time-dependent aspects of the process of magnetic reconnection, I argue that, for the physical conditions prevailing in blazar jets, the reconnection layer fragments leading to the formation a large number of plasmoids. Occasionally a plasmoid grows to become a large, “monster” plasmoid. I show that radiation emitted from the reconnection event can account for the observed “envelope” of ~day-long blazar activity while radiation from monster plasmoids can power the fastest observed TeV flares. The model is applied to several blazars with observed fast flaring. The inferred distance of the dissipation, blazar, zone is R~1-3 light-years. The required magnetization of the jet at this distance is sigma=B^2/4 pi rho c^2~ a few.