Particle acceleration in relativistic magnetic flux-merging events

Abstract: Using analytical and numerical methods (fluid and particle-in-cell simulations) we study a number of model problems involving merger of magnetic flux tubes in relativistic magnetically-dominated plasma. Mergers of current-carrying flux tubes (exemplified by the two dimensional `ABC' structures) and zero total current magnetic flux tubes are considered. In all cases regimes of spontaneous and driven evolution are investigated. We identify two stages of particle acceleration during flux mergers: (i) fast explosive prompt X-point collapse and (ii) ensuing island merger. The fastest acceleration occurs during the initial catastrophic X-point collapse, with the reconnection electric field of the order of the magnetic field. During the X-point collapse particles are accelerated by charge-starved electric fields, which can reach (and even exceed) values of the local magnetic field. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization $\sigma$. For plasma magnetization $\sigma \leq 10^2$ the spectrum power law index is $p> 2$; in this case the maximal energy depends linearly on the size of the reconnecting islands. For higher magnetization, $\sigma \geq 10^2$, the spectra are hard, $p< 2$, yet the maximal energy $\gamma_{max}$ can still exceed the average magnetic energy per particle, $ \sim \sigma$, by orders of magnitude (if $p$ is not too close to unity). The X-point collapse stage is followed by magnetic island merger that dissipates a large fraction of the initial magnetic energy in a regime of forced magnetic reconnection, further accelerating the particles, but proceeds at a slower reconnection rate.