A1 Refereed original research article in a scientific journal
First detection of X-ray polarization from the long-period X-ray pulsar 4U 1954+319; 
Authors: Salganik, Alexander; Kong, Lingda; Forsblom, Sofia V.; Zhou, Menglei; Liu, Honghui; Zhao, QingChang; Tsygankov, Sergey S.; Santangelo, Andrea; Poutanen, Juri
Publisher: EDP Sciences
Publication year: 2026
Journal: Astronomy and Astrophysics
Article number: A36
Volume: 710
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202558439
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1051/0004-6361/202558439
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/527083762
Self-archived copy's licence: CC BY
Self-archived copy's version: Publisher`s PDF
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 Pspin = 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.
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Funding information in the publication:
This work reports observations obtained with the Imaging X-ray Polarimetry Explorer (IXPE), a joint US (NASA) and Italian (ASI) mission, led by Marshall Space Flight Center (MSFC). The research uses data products provided by the IXPE Science Operations Center (MSFC), using algorithms developed by the IXPE Collaboration (MSFC, Istituto Nazionale di Astrofisica – INAF, Istituto Nazionale di Fisica Nucleare – INFN, ASI Space Science Data Center – SSDC), and distributed by the High-Energy Astrophysics Science Archive Research Center (HEASARC). This research also has made use of the NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the ASI Science Data Centre (ASDC, Italy) and Caltech. We are grateful to the IXPE and NuSTAR teams for the approval and rapid scheduling of the observations. AS acknowledges support from the Jenny and Antti Wihuri Foundation (grant no. 00240331). This research was supported by the International Space Science Institute (ISSI) in Bern, through International Team project 25-657 ‘Polarimetric Insights into Extreme Magnetism’ and the Research Council of Finland Centre of Excellence in Neutron-Star Physics (grant 374064). We thank the anonymous referee for their careful reading of the paper and thoughtful suggestions, which helped improve the clarity of the results.
