First detection of X-ray polarization from the long-period X-ray pulsar 4U 1954+319

TL;DR

This study reports the first detection of X-ray polarization from the long-period X-ray pulsar 4U 1954+319 using IXPE, a system where a slowly rotating neutron star accretes from a red supergiant wind. While the phase-averaged emission shows no significant polarization (MDP$_{99}=5.0\%$ in 2–8 keV), phase-resolved analysis reveals a polarization degree of $10.3\pm3.1\%$ at pulse maximum with a smooth PA rotation of ~$140^{\circ}$. By fitting the PA variation with the rotating vector model and performing event-by-event derotation, the phase-averaged polarization becomes evident at $5.2\sigma$ with $6.8\pm1.3\%$ PD in 2–8 keV, and energy-resolved sub-band PDs show no strong energy dependence within uncertainties. The analysis yields two plausible pulsar geometries, with the unbinned approach favoring one solution, highlighting the geometry of wind-fed accretion in this rare XRP class and demonstrating IXPE's capacity to constrain magnetospheric emission regions in such systems.

Abstract

We report the first detection of X-ray polarization with the Imaging X-ray Polarimetry Explorer (IXPE) from the X-ray pulsar (XRP) 4U 1954+319. The source belongs to an extremely rare class of systems in which a slowly rotating neutron star accretes from the dense wind of a red supergiant companion. Coherent pulsations are detected at $P_{\rm spin}=5.49\pm0.05$ h, which is one of the longest spin periods known among XRPs. While the phase-averaged analysis shows no significant polarization, with a 99% confidence minimum detectable polarization (MDP$_{99}$) of 5.0% in the 2-8 keV band, the phase-resolved analysis shows one interval at pulse maximum in which the polarization degree (PD) exceeds its MDP$_{99}$, giving ${\rm PD}=10.3\pm3.1$%. The polarization angle (PA) exhibits a smooth $\approx140^{\circ}$ rotation over the pulse, and a joint evaluation of all phase bins yields an overall detection significance of $3.3σ$. Using the rotating vector model, we identify the geometric solution that reproduces the observed PA variation. By subsequently applying an event-by-event derotation of the Stokes parameters based on this solution, we remove the PA swing and recover the phase-averaged polarization which is detected at the $5.2σ$ level.

Figures (5)

• Figure 1: $\chi^2$ periodogram of the 2--8 keV light curve. The best-fit period is marked with the vertical dashed line.
• Figure 2: Phase-resolved properties of 4U 1954$+$319 over two pulse cycles. The PD and PA measurements are derived from two methods: blue points show the results of the spectro-polarimetric analysis, and square red symbols show model-independent measurements obtained with ixpepolarization. Panels show (a) the normalized pulse profile in the 2--8 keV band, (b) and (c) the Stokes parameters $q$ and $u$, (d) the PD, and (e) the PA. The PA panel shows two RVM solutions from Table \ref{['tab:rvm_solutions']}, corresponding to two distinct posterior clusters shown in Fig. \ref{['fig:corner_rvm']}, solution 1 in orange and solution 2 in green.
• Figure 3: Protractor plots showing phase-resolved PD (radius) and PA (azimuth) in the 2--8 keV range. Contours indicate 1, 2, and 3$\sigma$ CLs for two parameters, shown in red, blue, and green, respectively. The cross marks the best-fit point, and the line shows the polarization direction.
• Figure 4: Posterior distributions of the RVM parameters derived from the phase-resolved PAs (spectro-polarimetric analysis). The contours correspond to 1$\sigma$, 2$\sigma$, and 3$\sigma$ CLs. The vertical lines in the diagonal panels mark the median values of the two posterior clusters.
• Figure 5: Posterior distributions of the RVM parameters obtained from the unbinned analysis. The contours correspond to the 1$\sigma$, 2$\sigma$, and 3$\sigma$ CLs. The vertical lines in the diagonal panels indicate the median values of the two posterior clusters.