The extent of power-law energy spectra in collisionless relativistic magnetic reconnection in pair plasmas

Using two-dimensional particle-in-cell simulations, we characterize the energy spectra of particles accelerated by relativistic magnetic reconnection (without guide field) in collisionless electron-positron plasmas, for a wide range of upstream magnetizations $\sigma$ and system sizes $L$. The particle spectra are well-represented by a power law $\gamma^{-\alpha}$, with a combination of exponential and super-exponential high-energy cutoffs, proportional to $\sigma$ and $L$, respectively. For large $L$ and $\sigma$, the power-law index $\alpha$ approaches about 1.2.