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    "story_id": "mp-2026-09-19-027",
    "source_story_id": "tmp-story-neutron-star-atmosphere-mismatch-front-rail",
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    "position": 16,
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    "headline": "The Wrong Atmosphere Could Hide a Biased Neutron-Star Radius",
    "slug": "the-wrong-atmosphere-could-hide-a-biased-neutron-star-radius",
    "dek": "At 20× stronger spot contrast, hydrogen-versus-helium mismatch shifted radius estimates while leaving acceptable residuals.",
    "summary": "At 20× stronger spot contrast, hydrogen-versus-helium mismatch shifted radius estimates while leaving acceptable residuals.",
    "body_text": "Synthetic NICER pulse data modeled on the roughly 2.1-solar-mass pulsar PSR J0740+6620 showed little radius bias from choosing hydrogen instead of helium, or vice versa, at the observed spot-to-background ratio. When the ratio increased about twentyfold with total counts held near 550,000, the wrong atmosphere produced significant radius bias while still fitting with statistically acceptable residuals. Bayesian evidence consistently preferred the correct composition, making model comparison—not residual inspection alone—the safeguard.",
    "why_it_matters": "At 20× stronger spot contrast, hydrogen-versus-helium mismatch shifted radius estimates while leaving acceptable residuals.",
    "limitations": [
      "Bayesian evidence consistently preferred the correct composition, making model comparison—not residual inspection alone—the safeguard."
    ],
    "importance": 9,
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    "json_url": "https://themachinepress.com/story/mp-2026-09-19-027.json",
    "first_published_at": "2026-09-19T09:00:00.000-04:00",
    "modified_at": "2026-09-19T09:00:00.000-04:00",
    "content_status": "new",
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        "text": "At 20× stronger spot contrast, hydrogen-versus-helium mismatch shifted radius estimates while leaving acceptable residuals.",
        "source_ids": [
          "source-2026-09-19-016"
        ],
        "qualification": "Bayesian evidence consistently preferred the correct composition, making model comparison—not residual inspection alone—the safeguard."
      }
    ],
    "source_ids": [
      "source-2026-09-19-016"
    ],
    "tags": [
      "neutron stars",
      "NICER",
      "atmosphere models"
    ],
    "image_url": null,
    "corrections": []
  },
  "sources": [
    {
      "source_id": "source-2026-09-19-016",
      "title": "arXiv preprint 2609.20795",
      "publisher": "arXiv",
      "url": "https://arxiv.org/abs/2609.20795",
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      "source_type": "primary_research",
      "is_primary_source": true,
      "published_at": "2026-09-17T13:54:31.000-04:00",
      "accessed_at": "2026-09-19T08:30:21.326-04:00",
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  "corrections": [],
  "publisher": {
    "name": "The Machine Press",
    "url": "https://themachinepress.com",
    "description": "A daily newspaper for the age of artificial intelligence."
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  "cite_this_report": {
    "title": "The Wrong Atmosphere Could Hide a Biased Neutron-Star Radius",
    "publisher": "The Machine Press",
    "published_at": "2026-09-19T09:00:00.000-04:00",
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