Lepton Acceleration by Relativistic Collisionless Magnetic Reconnection

We have calculated self-consistent equilibria of a collisionless relativistic electron-positron gas in the vicinity of a magnetic $X-$point. For the considered conditions, pertinent to extra-galactic jets, we find that leptons are accelerated up to Lorentz factors $\Gamma_0 = \kappa e B_0L {\cal E}^2 /mc^2 \gg 1$, where $B_0$ is the typical magnetic field strength, ${\cal E}\equiv E_0/B_0$, with $E_0$ the reconnection electric field, $L$ is the length scale of the magnetic field, and $\kappa \approx 12$. The acceleration is due to the dominance of the electric field over the magnetic field in a region around the $X-$point. The distribution function of the accelerated leptons is found to be approximately $dn/d\gamma \propto \gamma^{-1}$ for $\gamma \lesssim \Gamma_0$. The apparent distribution function may be steeper than $\gamma^{-1}$ due to the distribution of $\Gamma_0$ values and/or the radiative losses. Self-consistent equilibria are found only for plasma inflow rates to the $X-$point less than a critical value.Comment: submitted to ApJ; 6 figure