{
  "abstract": "We characterize long-term hardness behavior in 29 ultraluminous X-ray sources (ULXs) using over 3500 archival Swift/XRT observations. We classify observations as soft-band dominated, hard-band dominated, or uncertain from the hardness ratio (HR; 1.5--10/0.3--1.5 keV count rates) and its confidence bounds. We derive $f_{\\rm soft}$ and $f_{\\rm hard}$ and define the Crossing Index (CI) as the fraction of consecutive confidently classified observations on opposite sides of $HR=1$. We analyze these descriptors 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; clustering yields a similar three-group structure. Known or candidate accretor types tend to occupy band-dominated groups, whereas unknown accretors are more common in the mixed-band group. All four sources with reported long-term X-ray modulation discussed in an orbital or super-orbital context are classified as mixed-band. The three-cluster solution is preferred in 25 of 29 leave-one-out trials, while bootstrap tests find the band-dominated groups more stable than the mixed-band group. As the classification uses observed count rates, the groups represent Swift/XRT signatures rather than intrinsic spectral states. Response-folding tests show that modeled HR depends on absorption column density and photon index. Bayesian reanalysis of a representative low-count source reduces CI from 56.3\\% to 38.5\\% while preserving its mixed-band classification. Random downsampling produces no systematic shift in CI; halving the observations changes median CI by at most $\\sim10\\%$ across four sources. Larger samples with independent absorption constraints can test links among these observational classes, accretor type, long-term periodic modulation, and other X-ray properties.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.31324",
  "author_total": 5,
  "authors": [
    {
      "name": "Sinan Allak"
    },
    {
      "name": "Wei Yu"
    },
    {
      "name": "Menglei Zhou"
    },
    {
      "name": "Honghui Liu"
    },
    {
      "name": "Andrea Santangelo"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/0vxg4mdgzn81h.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.31324",
  "fulltext_url": null,
  "id": "0vxg4mdgzn81h",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.31324",
  "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": null,
  "pdf_url": "https://arxiv.org/pdf/2609.31324",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-25",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/0vxg4mdgzn81h.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:48Z",
      "native_id": "2609.31324",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.31324"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "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": "preprint",
  "updated_at": "2026-10-30T12:25:48Z",
  "url": "https://research.draconitic.com/paper/0vxg4mdgzn81h",
  "venue": "arXiv",
  "versions": [
    {
      "doi": "10.1051/0004-6361/202662175",
      "pid": "64f0fjntbnns3",
      "relation": "published-as",
      "title": "A statistical hardness classification of ultraluminous X-ray sources using long-term Swift /XRT monitoring",
      "venue": "Astronomy & Astrophysics"
    }
  ]
}