Testing Magnetic Field Configurations in Spider Pulsar PSR J1723−2837 with IXPE

Abstract We present the first X-ray polarimetry observations of a redback millisecond pulsar binary, PSR J1723−2837, with the Imaging X-ray Polarimetry Explorer (IXPE). We conduct a spectropolarimetric analysis combining IXPE data with archival Chandra, XMM-Newton, NuSTAR, and Swift observations. We explore two limiting magnetic field configurations, parallel and perpendicular to the bulk flow, and simulate their expected polarization signatures using the 3DPol radiative transport code. To account for the rapid rotation of the polarization angle (PA) predicted by these models, we implement a phase-dependent Stokes alignment procedure that preserves the polarization degree (PD) while correcting for a phase-rotating PA. We also devise a new maximum likelihood fitting strategy to determine the phase-dependence of the PA by minimizing the PD uncertainty. This technique hints that the binary may be rotating clockwise relative to the celestial north pole. We find no significant detection of polarization in the IXPE data, with PD ≲50% at the 99% confidence level. Our results exclude the high-PD scenario predicted by the perpendicular field model during the brightest orbital phase bin. Simulations show that doubling the current exposure would make the parallel configuration detectable. The new PA rotation technique is also applicable to IXPE data of many sources whose intrinsic PA variation is a priori not known but is strictly periodic.