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

Abstract

We report the first detection of X-ray polarization with the Imaging X-ray Polarimetry Explorer 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. We detect coherent pulsations at $P_{\mathrm{spin}}=5.49\pm 0.05$ 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 a single interval at pulse maximum in which the polarization degree (PD) exceeds the MDP${}_{99}$, yielding $\mathrm{PD}=10.2_{-3.0}^{+3.1}$%. The polarization angle (PA) exhibits a smooth $\approx 150^{\circ }$ 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. From this model, we infer a phase-independent $\mathrm{PD}$ of $6.1\pm 1.1$% in the 2-8 keV band.

Accepted to A&A. 7 pages, 7 figures, 3 tables