{
  "abstract": "We present the first sample-level search for supermassive black hole binaries (SMBHBs) requiring coherent quasi-periodicity at a common period in the X-ray and optical bands, over 1194 Swift-BAT hard X-ray AGN (Stage 1) and 175 4XMM-DR14 AGN (Stage 2). No source is a co-periodic candidate. Each light curve is modelled as a damped random walk (DRW) and searched with a Lomb-Scargle periodogram and a look-elsewhere-corrected Monte-Carlo significance. Because DRW red noise is largely independent between corona and disc, we require both bands individually significant with periods coincident within 5%, and gate the survivors with the model-independent null-signal-template test of Robnik et al. (2024). Over $P=100$-$900$ d the completeness-corrected 95% upper limit on the co-periodic fraction is amplitude-dependent: $\\lesssim 3\\%$ for hard-X-ray fractional modulation $\\gtrsim 0.3$, $\\approx 15\\%$ (precision-limited) at 0.2, and uninformative below $\\sim 0.15$ (the $ε=1$ floor is 0.25%). The sensitivity is set by the hard X-ray monitoring, not the optical photometry or the statistics, the opposite of the usual assumption. Daily MAXI and RXTE/ASM monitoring of the brightest AGN raises the X-ray completeness 5-8-fold, and the search remains null. Integrated over the BAT black-hole mass function, the expected all-amplitude co-periodic fraction is $\\sim 3\\times10^{-2}\\, f_{\\rm bin}\\, δ_{\\rm mod}$ (modulo a factor $g<1$), with $f_{\\rm bin}$ the sub-pc binary fraction, $δ_{\\rm mod}$ the modulating duty cycle, and $g$ the fraction reaching recoverable hard-X-ray amplitude, so a null is expected. We deliver a validated cross-band framework and the first completeness-corrected constraint on the co-periodic fraction, ready for the denser X-ray monitoring of Einstein Probe and eROSITA.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": "Journal reference: Astrophys. J. 1009 (2026) 184",
  "arxiv": "2608.16787",
  "author_total": 1,
  "authors": [
    {
      "name": "Karan Akbari"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2g9620eys2ygt.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2608.16787",
  "fulltext_url": null,
  "id": "2g9620eys2ygt",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2608.16787",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ],
    [
      "arxiv",
      "astro-ph.HE"
    ],
    [
      "arxiv",
      "astro-ph.IM"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2608.16787",
  "pmcid": null,
  "pmid": null,
  "published": "2026-08-18",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/2g9620eys2ygt.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:25:31Z",
      "native_id": "2608.16787",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2608.16787"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.IM",
      "subfield": "instrumentation-methods"
    }
  ],
  "title": "A Cross-Band (X-ray $\\times$ Optical) Periodicity Search for Supermassive Black Hole Binaries: A Null Result and the First Completeness-Corrected Constraint",
  "title_text": "A Cross-Band (X-ray × Optical) Periodicity Search for Supermassive Black Hole Binaries: A Null Result and the First Completeness-Corrected Constraint",
  "type": "preprint",
  "updated_at": "2026-10-30T12:25:31Z",
  "url": "https://research.draconitic.com/paper/2g9620eys2ygt",
  "venue": "arXiv",
  "versions": [
    {
      "doi": "10.3847/1538-4357/ae9d7c",
      "pid": null,
      "relation": "published-as",
      "title": null,
      "venue": null
    }
  ]
}