Particle Acceleration and Nonthermal Emission at the Intrabinary Shock of Spider Pulsars. I: Non-Radiative Simulations

Abstract: Spider pulsars are compact binary systems composed of a millisecond pulsar and a low-mass companion. Their X-ray emission - modulated on the orbital period - is interpreted as synchrotron radiation from high-energy electrons accelerated at the intrabinary shock. We perform global two-dimensional particle-in-cell simulations of the intrabinary shock, assuming that the shock wraps around the companion star. When the pulsar spin axis is nearly aligned with the orbital angular momentum, we find that the magnetic energy of the relativistic pulsar wind - composed of magnetic stripes of alternating field polarity - efficiently converts to particle energy at the intrabinary shock, via shock-driven reconnection. The highest energy particles accelerated by reconnection can stream ahead of the shock and be further accelerated by the upstream motional electric field. In the downstream, further energization is governed by stochastic interactions with the plasmoids / magnetic islands generated by reconnection. We also extend our earlier work (Cort\'es & Sironi 2022) by performing simulations that have a larger (and more realistic) companion size and a more strongly magnetized pulsar wind. We confirm that our first-principles synchrotron spectra and lightcurves are in good agreement with X-ray observations.