{
  "abstract": "Primordial black holes (PBHs) are expected to be born essentially non-spinning and, if the primordial curvature fluctuations possess non-Gaussian tails, strongly clustered. Any spin measured in a PBH population must therefore be dynamically generated. Using direct $N$-body simulations with \\textsc{nbody6++gpu}, modified to treat nearly parabolic dynamical-capture mergers, we quantify the conditions under which an initially non-spinning PBH cluster develops the spin, mass and stochastic-background signatures that gravitational-wave observations are now sensitive to. We find a sharp tension: producing a root-mean-square spin $χ_{\\rm rms}\\sim0.1 - 0.2$ in the $30 - 50\\,M_\\odot$ range requires initial Plummer radii of order $10^{-5}\\,{\\rm pc}$, whereas survival over cosmological times requires parsec-scale clusters. This tension is resolved naturally if PBHs form with a compact, short-lived core embedded in an extended halo, and we therefore simulate the two regimes as independent blocks. The core block ($4000$ PBHs, Plummer radii $3\\times10^{-6} - 10^{-4}\\,{\\rm pc}$) collapses in days to years, producing a burst of mergers whose orbits are strongly non-circular ($e\\to1$) and of low dimensionless orbital angular momentum, so that remnant spins are systematically below the quasi-circular value $χ\\simeq0.7$. Redshifted from their formation epoch, the core mergers deposit a stochastic background peaking in the $10^{-6}$--$10^{-4}\\,{\\rm Hz}$ band accessible to LISA. The picture predicts a specific division of labour between observables, spins set in the primordial collapse, detectable mergers set by the long-term halo, and a two-component stochastic background, and, in particular, that both the spin and the mass data favour a primordial mass function truncated near $50\\,M_\\odot$.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.01331",
  "author_total": 2,
  "authors": [
    {
      "name": "Marc Barceló-Sastre"
    },
    {
      "name": "Juan García-Bellido"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/dy71621k8gmjw.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.01331",
  "fulltext_url": null,
  "id": "dy71621k8gmjw",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.01331",
  "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.CO"
    ],
    [
      "arxiv",
      "gr-qc"
    ],
    [
      "openalex",
      "3103|Pulsars and Gravitational Waves Research"
    ]
  ],
  "openalex": "W7206192707",
  "pdf_url": "https://arxiv.org/pdf/2609.01331",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-01",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/dy71621k8gmjw.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:00Z",
      "native_id": "2609.01331",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.01331"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.CO",
      "subfield": "cosmology"
    },
    {
      "field": "physics",
      "origin": "arxiv:gr-qc",
      "subfield": "gravitation"
    }
  ],
  "title": "Gravitational-wave signatures of primordial black hole clusters and the imprint of an early dense-core collapse",
  "title_text": "Gravitational-wave signatures of primordial black hole clusters and the imprint of an early dense-core collapse",
  "type": "preprint",
  "updated_at": "2026-10-30T12:18:00Z",
  "url": "https://research.draconitic.com/paper/dy71621k8gmjw",
  "venue": "arXiv",
  "versions": []
}