{
  "abstract": "We present a homogeneous statistical framework to characterize observed long-term hardness behavior in 29 ultraluminous X-ray sources using over 3500 archival Swift X-Ray Telescope (XRT) observations. Observations were classified as soft-band dominated, hard-band dominated, or uncertain using the hardness ratio and its confidence bounds. We defined HR as the ratio of the 1.5−10 to 0.3−1.5 keV count rates. We derived f soft and f hard and introduced the crossing index (CI), defined as the fraction of consecutive pairs of confidently classified observations that fall on opposite sides of the HR = 1 boundary. These descriptors were analyzed with rule-based classification and unsupervised hierarchical clustering. The rule-based analysis identifies eight soft-band-dominated, five hard-band-dominated, and 16 mixed-band systems, while clustering reveals a similar three-group structure. Sources with known or candidate accretor types typically occupy band-dominated groups, whereas unknown accretors are more common in the mixed-band group. All four sources with reported long-term X-ray modulations discussed in an orbital or super-orbital context belong to the rule-based mixed-band class. The three-cluster solution is preferred in 25 of 29 leave-one-out trials, while bootstrap tests reveal the band-dominated groups to be more stable than the mixed-band group. Because the classification uses observed count rates, these groups represent Swift /XRT signatures rather than intrinsic spectral states. Response-folding tests show that the modeled hardness ratio depends on absorption column density and photon index. Bayesian reanalysis of a representative low-count source reduced CI from 56.3% to 38.5% but preserved its mixed-band classification. Random downsampling produced no systematic shift in CI, using half of the observations changed median CI by at most ∼10% across four sources. Larger samples with independent absorption constraints can test links between these observational classes, accretor type, long-term periodic modulation, and other X-ray properties.",
  "abstract_shown": true,
  "affiliations": [
    "Institut für Astronomie und Astrophysik"
  ],
  "article_type": null,
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  "author_total": 5,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Sinan Allak",
      "orcid": "0000-0001-7093-1079"
    },
    {
      "affs": [
        0
      ],
      "name": "Wei Yu",
      "orcid": "0000-0002-3229-2453"
    },
    {
      "affs": [
        0
      ],
      "name": "Menglei Zhou",
      "orcid": "0000-0001-8250-3338"
    },
    {
      "affs": [
        0
      ],
      "name": "Honghui Liu",
      "orcid": "0000-0003-2845-1009"
    },
    {
      "affs": [
        0
      ],
      "name": "Andrea Santangelo",
      "orcid": "0000-0003-4187-9560"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/64f0fjntbnns3.bib",
  "cited_by": 0,
  "doi": "10.1051/0004-6361/202662175",
  "fulltext_url": "https://www.aanda.org/articles/aa/full_html/2026/11/aa62175-26/aa62175-26.html",
  "id": "64f0fjntbnns3",
  "issns": [
    "0004-6361",
    "1432-0746"
  ],
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  "language": "en",
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0",
  "metadata_licence": null,
  "native_subjects": [
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      "lcc",
      "QB1-991"
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    [
      "openalex",
      "3103|Astrophysical Phenomena and Observations"
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  "openalex": "W7214573892",
  "pdf_url": null,
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-28",
  "publisher": "EDP Sciences",
  "ris": "https://research.draconitic.com/paper/64f0fjntbnns3.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:18Z",
      "native_id": "10.1051/0004-6361/202662175",
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      "url": "https://www.aanda.org/10.1051/0004-6361/202662175"
    },
    {
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  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "journal-openalex:3103|Stellar, planetary, and galactic studies",
      "subfield": "stellar"
    }
  ],
  "title": "A statistical hardness classification of ultraluminous X-ray sources using long-term Swift /XRT monitoring",
  "title_text": "A statistical hardness classification of ultraluminous X-ray sources using long-term Swift /XRT monitoring",
  "type": "peer-reviewed",
  "updated_at": "2026-10-31T14:22:36Z",
  "url": "https://research.draconitic.com/paper/64f0fjntbnns3",
  "venue": "Astronomy & Astrophysics",
  "versions": [
    {
      "doi": "10.48550/arxiv.2609.31324",
      "pid": "0vxg4mdgzn81h",
      "relation": "preprint",
      "title": "A statistical hardness classification of ultraluminous X-ray sources using long-term Swift/XRT monitoring",
      "venue": "arXiv"
    }
  ]
}