Revisiting the Intrabinary Shock Model for Millisecond Pulsar Binaries: Radiative Losses and Long-Term Variability

Abstract

Spectrally hard x-ray emission with double-peak light curves (LCs) and orbitally modulated gamma rays have been observed in some millisecond pulsar binaries, phenomena attributed to intrabinary shocks (IBSs). While the existing IBS model by Sim et al. (2024; Astrophysical Journal 964(2):109) successfully explains these high-energy features observed in three pulsar binaries, it neglects particle energy loss within the shock region. We refine this IBS model to incorporate radiative losses of x-ray emitting electrons and positrons and verify that the losses have insignificant impact on the observed LCs and spectra of the three binaries. Applying our refined IBS model to the x-ray bright pulsar binary PSR J1723-2837, we predict that it can be detected by the Cherenkov Telescope Array. Additionally, we propose that the long-term x-ray variability observed in XSS J12270-4859 and PSR J1723-2837 is due to changes in the shape of their IBSs. Our modeling of the x-ray variability suggests that these IBS shape changes may alter the extinction of the companion's optical emission, potentially explaining the simultaneous optical and x-ray variability observed in XSS J12270-4859. We present the model results and discuss their implications.