[2509.19446] Vacuum birefringence and the polarized X-ray emission from a radio magnetar

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arXiv:2509.19446 (astro-ph)

[Submitted on 23 Sep 2025 (v1), last revised 6 Aug 2026 (this version, v4)]

Title:Vacuum birefringence and the polarized X-ray emission from a radio magnetar

Authors:Rachael E. Stewart, Hoa Dinh Thi, George Younes, Marcus E. Lower, Matthew G. Baring, Michela Negro, Fernando Camilo, Joel B. Coley, Teruaki Enoto, Alice K. Harding, Wynn C. G. Ho, Chin-Ping Hu, Philip Kaaret, Paul Scholz, Alex Van Kooten, Zorawar Wadiasingh<br>View a PDF of the paper titled Vacuum birefringence and the polarized X-ray emission from a radio magnetar, by Rachael E. Stewart and 15 other authors

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Abstract:Magnetars are isolated neutron stars with exceptionally strong surface fields exceeding $10^{14}$ G. Their bright X-ray emission probes physical regimes in which quantum electrodynamic (QED) influences radiation propagation. Strong magnetic fields induce polarization-dependent refractive indices in the vacuum; such vacuum birefringence (VB) remains a long-standing but unconfirmed prediction of QED. Here, we report phase- and energy-resolved polarization measurements of the radio-emitting magnetar 1E 1547.0$-$5408 obtained by coordinating X-ray and radio observations from the Imaging X-ray Polarimetry Explorer (IXPE), the Neutron Star Interior Composition ExploreR (NICER), and the Parkes/Murriyang observatory. We detect large polarization degrees (PD) in the thermally-dominant soft X-ray band, reaching phase-averaged values of $65\%$ at 2 keV before substantially decreasing between 2$-$4 keV. At certain rotational phases, the 2$-$3 keV PD rises to nearly $80\%$ while remaining high ($\gtrsim 40\%$) throughout the radio beam crossing. The phase-dependent X-ray and radio polarization angles are both consistent with the rotating vector model, suggesting that the emission geometries track the star's large-scale magnetic field. Collectively, these characteristics challenge standard surface emission models using non-refractive propagation of light to infinity. VB-governed magnetospheric propagation can naturally explain the X-ray polarization signals. Our results represent a significant advance in probing this hallmark prediction of QED, opening a new cosmic window into superstrong-field quantum physics, thereby motivating further observational and theoretical studies concentrating on this domain.

Comments:<br>This is a version of the submitted article. The Version of Record of this article is published in Nature (2026), and is available online at this https URL

Subjects:

High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Experiment (hep-ex); Quantum Physics (quant-ph)

Cite as:<br>arXiv:2509.19446 [astro-ph.HE]

(or<br>arXiv:2509.19446v4 [astro-ph.HE] for this version)

https://doi.org/10.48550/arXiv.2509.19446

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arXiv-issued DOI via DataCite

Related DOI:

https://doi.org/10.1038/s41586-026-10859-z

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Submission history<br>From: Hoa Dinh Thi [view email]<br>[v1]<br>Tue, 23 Sep 2025 18:01:21 UTC (2,710 KB)

[v2]<br>Fri, 27 Feb 2026 20:40:03 UTC (2,820 KB)

[v3]<br>Tue, 14 Jul 2026 22:43:55 UTC (2,574 KB)

[v4]<br>Thu, 6 Aug 2026 15:33:15 UTC (2,574 KB)

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View a PDF of the paper titled Vacuum birefringence and the polarized X-ray emission from a radio magnetar, by Rachael E. Stewart and 15 other authors<br>View PDF<br>HTML (experimental)<br>TeX Source

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