{
  "abstract": "Vacuum birefringence in a strong magnetic field is often modelled by combining the finite-field splitting for perpendicular propagation with the familiar $\\sin^2θ$ angular factor. That separation is exact only at leading response order. Continuing our previous study of the field-strength dependence of the birefringence, we examine the angular dependence of the full one-loop refractive indices in the local Heisenberg--Euler theory. We show analytically that, on the positive-birefringence branch, the usual factorized form systematically overestimates the exact splitting away from perpendicular propagation. The relative correction is largest close to the magnetic-field direction, where the absolute birefringence itself vanishes. Compact analytic expansions expose the sign, scaling, and angular location of the correction throughout the magnetar-relevant range $B/B_{\\rm cr}\\le100$, while an independent proper-time calculation verifies the underlying one-loop coefficients. In centred-dipole applications the correction is sub-percent for moderate magnetar fields but reaches several percent for the strongest sources; surface averaging and radial phase accumulation reduce its physical weight as the field decreases away from the stellar surface. The result separates finite-field amplitude and angular corrections and provides controlled analytic benchmarks for precision polarization transport in strongly magnetized environments.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.38573",
  "author_total": 2,
  "authors": [
    {
      "name": "Shahram Abbassi"
    },
    {
      "name": "S. R. Valluri"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/7190hfgts8zct.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.38573",
  "fulltext_url": null,
  "id": "7190hfgts8zct",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.38573",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.HE"
    ],
    [
      "openalex",
      "3107|Quantum Electrodynamics and Casimir Effect"
    ]
  ],
  "openalex": "W7215091136",
  "pdf_url": "https://arxiv.org/pdf/2609.38573",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-29",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/7190hfgts8zct.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:49Z",
      "native_id": "2609.38573",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.38573"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "Angular structure of finite-field vacuum birefringence in magnetars",
  "title_text": "Angular structure of finite-field vacuum birefringence in magnetars",
  "type": "preprint",
  "updated_at": "2026-10-30T12:25:49Z",
  "url": "https://research.draconitic.com/paper/7190hfgts8zct",
  "venue": "arXiv",
  "versions": []
}