MAGNETIC RECONNECTION IN RELATIVISTIC JETS

Relativistic jets are produced by accreting black holes around which accumulation of magnetic fields leads to relativistic magnetizations; they are the dominant feature of blazars, a class of active galactic nuclei. Blazars are defined by very broad and luminous spectral energy distributions (SEDs) that are both strongly (stochastically) variable and spectrally stable. The maximum energy of electrons (and positrons) producing these SEDs must be robustly regulated. The prevalent view is that the regulating factor is radiative cooling, however, this implies extremely weak electric fields relative to magnetic fields, E/B similar to 10(-9) (this is equivalent to the separation of Larmor and cooling time scales; this is also why the synchrotron SEDs of blazars extend to much lower energies than that of the Crab pulsar wind nebula). The alternative regulating factor could be local (highly inhomogeneous) magnetization. Relativistic magnetic reconnection has been proposed to explain rapid (a few minutes) gamma-ray flares of blazars. Two particular scenarios have been developed: minijets (Alfvenic outflows) and plasmoids (magnetic flux ropes). It was recently demonstrated that plasmoids are better suited to produce rapid flares (by tail-on mergers), because their higher density is a decisive advantage over the higher Lorentz factors of minijets. Magnetic reconnection can be triggered in relativistic jets by instabilities of toroidal magnetic fields, driven either by electric currents or by gas pressure, which were recently shown to lead to efficient particle acceleration.