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

We report the first detection of X-ray polarization with the from the X-ray pulsar (XRP) =5.49 =10.2_ Imaging X-ray Polarimetry Explorer 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. We detect coherent pulsations at P_ ̊m spin 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 a single interval at pulse maximum in which the polarization degree (PD) exceeds the MDP_99, yielding PD -3.0 ^ +3.1 %. The polarization angle (PA) exhibits a smooth approx150 $ of 6.1 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 a geometric solution that reproduces the observed PA variation. From this model, we infer a phase-independent $ ̊m PD in the 2--8 keV band.