Magnetospheric birefringence breaks the rotating-vector model and depolarises X-ray pulsars

[2608.27699] Magnetospheric birefringence breaks the rotating-vector model and depolarises X-ray pulsars Skip to main content Search Submit Donate Log in Astrophysics > High Energy Astrophysical Phenomena arXiv:2608.27699 (astro-ph) [Submitted on 27 Aug 2026] Title: Magnetospheric birefringence breaks the rotating-vector model and depolarises X-ray pulsars Authors: Alexander A. Mushtukov , Sergey S. Tsygankov , Silvia Zane , Nabil Brice , Ruth M. E. Kelly , Roberto Taverna , Roberto Turolla View PDF HTML (experimental) Abstract: Accreting X-ray pulsars have long been expected to show large linear polarisation below the cyclotron resonance. In the standard picture, this follows from two facts: radiative opacities in the strongly magnetised neutron star environment depend strongly on polarisation mode, and the emission comes from compact polar regions. The emerging radiation should therefore be locally polarised and add almost coherently, with a polarisation angle tracing the projected magnetic field as in the rotating-vector model. Recent X-ray polarimetric observations, instead, show modest polarisation fractions and position angle swings that often depart from this simple picture. Here we show that propagation through the magnetosphere of an accreting neutron star can naturally produce both effects. Using general-relativistic ray tracing and Stokes-parameter transport, we find that photons emitted from compact polar regions acquire ray-dependent polarisation angle offsets in two ways. First, nearly field-aligned trajectories can cross local non-adiabatic pockets inside the QED birefringent region. Second, photons that intersect the plasma-loaded accretion flow accumulate additional phase shifts between plasma normal modes. Both effects broaden the distribution of polarisation angles across the ray bundle and reduce the hotspot-integrated linear polarisation, while also producing systematic deviations from rotating-vector-model expectations. Magnetospheric propagation should therefore be treated as part of the polarimetric transfer problem in accreting X-ray pulsars, and may help explain the observed low polarisation fractions and anomalous angle swings. Comments: submitted to MNRAS, 17 pages, 10 figures Subjects: High Energy Astrophysical Phenomena (astro-ph.HE) ; Solar and Stellar Astrophysics (astro-ph.SR); Space Physics (physics.space-ph) Cite as: arXiv:2608.27699 [astro-ph.HE] (or arXiv:2608.27699v1 [astro-ph.HE] for this version) https://doi.org/10.48550/arXiv.2608.27699 Focus to learn more arXiv-issued DOI via DataCite Submission history From: Alexander A. Mushtukov [ view email ] [v1] Thu, 27 Aug 2026 20:35:28 UTC (5,031 KB) Full-text links: Access Paper: View PDF HTML (experimental) TeX Source view license Additional Features Audio Summary Current browse context: astro-ph.HE < prev | next > new | recent | 2026-08 Change to browse by: astro-ph astro-ph.SR physics physics.space-ph References & Citations INSPIRE HEP NASA ADS Google Scholar Semantic Scholar export BibTeX citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer ( What is the Explorer? ) Connected Papers Toggle Connected Papers ( What is Connected Papers? ) Litmaps Toggle Litmaps ( What is Litmaps? ) scite.ai Toggle scite Smart Citations ( What are Smart Citations? ) Code, Data, Media Code, Data and Media Associated with this Article alphaXiv Toggle alphaXiv ( What is alphaXiv? ) Links to Code Toggle CatalyzeX Code Finder for Papers ( What is CatalyzeX? ) DagsHub Toggle DagsHub ( What is DagsHub? ) GotitPub Toggle Gotit.pub ( What is GotitPub? ) Huggingface Toggle Hugging Face ( What is Huggingface? ) ScienceCast Toggle ScienceCast ( What is ScienceCast? ) Demos Demos Replicate Toggle Replicate ( What is Replicate? ) Spaces Toggle Hugging Face Spaces ( What is Spaces? ) Spaces Toggle TXYZ.AI ( What is TXYZ.AI? ) Related Papers Recommenders and Search Tools Link to Influence Flower Influence Flower ( What are Influence Flowers? ) Core recommender toggle CORE Recommender ( What is CORE? ) IArxiv recommender toggle IArxiv Recommender ( What is IArxiv? ) Author Venue Institution Topic About arXivLabs arXivLabs: experimental projects with community collaborators arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website. 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