X-ray Polarization of the Intrabinary Shock in Redback Pulsar J1723$-$2837

TL;DR

The study tests the magnetic-field geometry of the intrabinary shock in the redback pulsar J1723-2837 using X-ray polarization measurements from IXPE. It employs an advanced, unbinned maximum-likelihood analysis with orbital-phase resolution and careful background modeling to separate a modulated IBS component from an unmodulated, potentially unpulsed component. The results favor a CCW striped-wind IBS geometry, with an IBS polarization around 36% and an EVPA near 0°, but the detection is not statistically decisive (ΔAIC=0 relative to an unpolarized model) and the total-polarization upper limit is 36% at 99% confidence. Overall, the findings are consistent with striped-wind simulations and demonstrate the potential of phase-resolved X-ray polarization to probe pulsar-wind magnetization, while highlighting the need for deeper exposures to achieve a robust detection.

Abstract

The intrabinary shocks (IBS) in spider pulsars emit non-thermal synchrotron X-rays from accelerated electrons and positrons in the shocked pulsar wind, likely energized by magnetic reconnection. The double-peaked X-ray light curves from these shocks have been well characterized in several spider systems. In this paper, we analyze Imaging X-ray Polarimetry Explorer (IXPE) observations of the redback pulsar J1723$-$2837 to examine the expected synchrotron polarization. Using advanced extraction methods that include spatial, temporal, and particle background weights, we constrain the polarization of the IBS. We compare different models for the magnetic field in the radiation zone and find that the best fit prefers a striped pulsar wind model over other polarized models, with maximum polarization degree of the IBS emission component $Π_{\rm IBS}=36^{+16}_{-15}\%$, in addition to an unpolarized non-IBS component. Since this is only 2.4$σ$, we cannot claim strong preference over an unpolarized model; we report a $99\%$ confidence level upper limit on the total polarization of both IBS and non-IBS components $Π_{99}<36\%$, which is improved over the $50\%$ limit obtained in previous work. The best-fit polarization of the IBS component is consistent with numerical simulations. Detailed tests of such models are accessible to future measurements.

Figures (4)

• Figure 1: Left: The measured IXPE light curve for J1723 folded on its orbital period. The dashed line shows the estimated level of the non-IBS unmodulated flux. Middle and right: The best fitting counterclockwise (CCW; blue) striped wind model polarization pattern from our unbinned analysis described in Sec. \ref{['sec:Model']}, as compared to the deprecated clockwise (CW) and CCW flow models.
• Figure 2: A 3D visualization of the IBS with the striped wind (blue) and flow (orange) magnetic field lines. The pulsar (red dot) and companion are also shown.
• Figure 3: Quiescent r-band K2 light curve (dominated by ellipsoidal modulation) and residuals to the best fit model. Errors are inflated from their originally statistical values to account for rapid modulation, plausibly due to low level companion flare activity.
• Figure 4: Shaded bands show the measured J1723 1$\sigma$ and 2$\sigma$ confidence regions for $\Pi_{\rm IBS}$. The $\Pi_{\rm IBS}$ values between the black dashed lines map to $\alpha_{\rm pw}$ values shown on the right axis using the linear version of eq. 10 from 2025ApJ...991...98S, for $i=60^\circ$. We show the phase-averaged-flux-weighted mean $\alpha_{\rm pw}$ across our model IBS as a function of binary viewing inclination $i$ for different magnetic obliquity angles $\alpha_B$ and mark our light curve-measured $i=40.3^\circ$. At $i<20^\circ$, the IBS emission is unlikely to be detected (see text), so we continue the curves with dashed-dotted lines.