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

TL;DR

This work presents the first X-ray polarimetry of a redback spider MSP, PSR J1723-2837, using IXPE combined with archival X-ray data to test two magnetic-field configurations (PAR and PER) for the intrabinary shock. It introduces a phase-dependent Stokes alignment and a maximum-likelihood PA-variation approach to extract phase-resolved polarization in the presence of rapid PA rotation. The analysis finds no significant polarization, excluding the extreme PER case at current sensitivity, and shows that doubling the exposure would likely detect the PAR polarization signature. The methods, particularly the phase-alignment technique, provide a broadly applicable framework for periodic PA variation in IXPE data and pave the way for deeper polarization constraints with future observations.

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 spectro-polarimetric 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 predicted by these models, we implement a phase-dependent Stokes alignment procedure that preserves the polarization degree while correcting for a phase-rotating PA. We also devise a new maximum-likelihood fitting strategy to determine the phase-dependence of the polarization angle by minimizing the polarization degree 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 99% confidence level. Our results excludes the high-polarization degree 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.

Figures (11)

• Figure 1: Left: Schematic illustration of an intrabinary pulsar wind shock in a compact binary system as seen by an observer at inclination $i=40^\circ$ with respect to the circular orbital plane. The pulsar launches a relativistic wind that is confined by the presence of a companion star, forming a shock surface (gray-ish cap). Red arrows indicate the PAR magnetic field configuration, while blue arrows mark the PER configuration. Right: Comparison between two polarization models for PSR J1723$-$2837 using 3DPol: the parallel (PAR) model shown in red, and the perpendicular (PER) model shown in blue. The top panel displays the pulse profile in arbitrary units, where both models are compared against real IXPE data points. The center panel shows the polarization fraction predicted by each model, with the PER model exhibiting a significantly higher modulation amplitude than the PAR model. The bottom panel presents the predicted polarization angle (PA) as a function of phase. The discontinuous jump in the polarization position angle occurs when the net polarization fraction approaches zero, at which point the PA becomes formally undefined and may change abruptly by 90°. In the PAR configuration, this condition is naturally reached at orbital phases 0 and 0.5 due to nearly complete cancellation of the polarized emission from different regions of the shocked flow. In the PER configuration, a similar behavior is in principle possible, but it is significantly more difficult to realize. In this case, Doppler boosting is intrinsically coupled to the magnetic field geometry, and the regions where polarization cancellation can occur are confined to the wings of the shock, where the emission is strongly de-boosted. As a result, the total polarized flux rarely approaches zero, and the PA therefore evolves smoothly with orbital phase for a wide range of parameter choices.
• Figure 2: Background subtracted X-ray light curves of PSR J1723$-$2837 obtained with XMM-Newton, Chandra, NuSTAR, and IXPE (from top to bottom, respectively) folded at the binary orbital ephemeris from crawford2013psr. The gap in the XMM-Newton light curve is due to incomplete coverage of the orbit. Orbital phase zero corresponds to the time of the ascending node of the pulsar. Two cycles are shown for clarity.
• Figure 3: Joint fit of the IXPE DU1--DU3 (cyan, magenta, and yellow), NuSTAR FPMA+FPMB (blue and green), Chandra ACIS-S (black), Swift XRT (orange), and XMM-Newton (red) with an absorbed powerlaw with a tied power-law index but independent normalizations. See Table \ref{['tab:fit']} for best fit parameters. The bottom panel shows the best fit residuals expressed in terms of standard deviations ($\sigma$).
• Figure 4: Top: Pulse profile of the simulated polarized and unpolarized component for the PER (left) and PAR (right) models. The real IXPE data pulse profile for PSR J1723$-$2837 is normalized to the maximum of the sum of the two components and overlaid for comparison. We make sure that the simulated observation carry the same statistics in the macro-phase bin [0.55-0.95] as the real one. Middle and bottom: phase-resolved polarization analysis of simulated IXPE exposures of $\times1$ (middle row) and $\times2$ (bottom row) of the current IXPE observation exposure ($\sim$ 1.254 Ms). In blue on the left column we illustrate the results for the PER model, while in red on right column we show the equivalent results for the PAR model.
• Figure 5: Results of the PCUBE analysis in the macro phase bins (0.05--0.45, 0.55--0.95) for the energy band 2--6 keV. The top panel shows the model-predicted polarization degree (PD) as a function of rotational phase for the perpendicular (blue) and parallel (red) configurations. The middle panel presents the pulse profile (arbitrary units), illustrating the different statistics in the two phase intervals used for polarization measurements. In the bottom panel, measured upper limits for the polarization degrees for both PAR (red) and PER (blue) configurations are shown. These are compared against the expected PD values from the models (dashed lines), while the shaded gray regions indicate the MDP$_{99}$.