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 .