High Energy Emission from the Intrabinary Shocks in Redback Pulsars

Abstract: The intrabinary shocks (IBS) of spider pulsars emit non-thermal synchrotron X-rays from accelerated electrons and positrons in the shocked pulsar wind, likely energized by magnetic reconnection. In redback spider pulsars, the IBS typically wraps around the pulsar, leading to a near-normal IBS shock with relatively bright X-ray emission. The characteristic energies of radiating particles and the magnetic fields in the IBS suggest spectral features in the hard X-ray band. Here we perform joint soft-hard X-ray analyses of three redback pulsars, J1723-2837, J2215+5135, and J2339-0533, including new J2215 NuSTAR data. We identify a significant cooling break in J1723-2837 and a marginal break in J2215+5135, while placing constraints on the break energy in J2339-0533. Interpreting these as synchrotron cooling features allows us to estimate the IBS magnetic field $B_{\rm IBS} \sim 40-100$ G and place lower bounds on the maximum radiating electron energy. Our results constrain the magnetization of the pulsar wind as well as pair-production in millisecond pulsar magnetospheres.