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 pulsar 4U 1954+319, revealing phase-dependent polarization features and providing insights into the magnetic geometry of the neutron star.

Contribution

First X-ray polarization detection from 4U 1954+319, demonstrating phase-resolved polarization and modeling the magnetic geometry of this rare long-period pulsar.

Findings

Phase-resolved polarization exceeds 5% at pulse maximum.

Polarization angle shows a smooth 140° rotation over the pulse.

Phase-averaged polarization detected at 5.2σ significance.

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 $3.3\sigma$. 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 $5.2\sigma$ level.