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

¹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
² Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
⁴ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 December 2025
Accepted: 19 March 2026

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_spin = 5.49 ± 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 (MDP₉₉) 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 MDP₉₉, yielding PD = 10.2^+3.1_−3.0%. The polarization angle (PA) exhibits a smooth ≈150° 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 PD of 6.1 ± 1.1% in the 2–8 keV band.