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Description
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 4.9% 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 ${\rm PD}=10.2_{-3.0}^{+3.1}$%. The polarization angle (PA) exhibits a smooth $\approx150^{\circ}$ rotation over the pulse, and a joint evaluation of all phase bins yields an overall detection significance of $3.3\sigma$. Using the rotating vector model, we identify a geometric solution that reproduces the observed PA variation and measure a phase-independent ${\rm PD}$ of $6.1\pm1.1$% in the 2$-$8 keV band, corresponding to a $5.5\sigma$ detection.