{
  "abstract": "The microlensing optical depth towards the Large Magellanic Cloud (LMC), $τ_{\\rm LMC}$, bounds the compact-object fraction of the Galactic dark matter halo, but is almost always evaluated for a spherical halo. We compute it for triaxial haloes and show that the shape dependence is carried entirely by the projection of the line of sight onto the halo principal axes: for a scale-free halo $ρ\\propto R^{-α}$ at fixed local density it factorises exactly as $τ/τ_{\\rm sph}=S_\\odot^α\\langle S^{-α}\\rangle_w$, with $S(Ω)$ the ellipsoidal shape function. The LMC lies only $0\\fdg78$ from perpendicular to the Sun-Galactic Centre line, making it a near-optimal probe of in-plane elongation and a poor probe of vertical flattening. For the halo shapes favoured by \\textit{Gaia} we find $τ_{\\rm LMC}/τ_{\\rm sph}=0.95$--$1.15$ at fixed local density, $1.57$ for the Law-Majewski logarithmic potential, and $0.67$--$1.54$ when the major-axis azimuth is left free. For the Einasto halo required by the declining \\textit{Gaia} DR3 rotation curve, truncation near $25\\kpc$ is offset by the larger local density it implies and $τ$ falls by only $7$--$15$ per cent; but the median lens distance moves inwards from $13.4$ to $8.5\\kpc$, where the logarithmic slope is steep and rising, amplifying the shape sensitivity: the \\textit{Gaia} RR~Lyrae enhancement grows from $15$ to $23$ per cent and the orientation-free range widens to $0.62$--$1.62$. The systematic on the inferred MACHO or primordial-black-hole fraction is two-sided, of order $\\pm15$ per cent for the measured shapes and a factor $2.6$ under full orientation freedom, exceeding the statistical errors of the EROS-2 and OGLE bounds.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.08988",
  "author_total": 2,
  "authors": [
    {
      "name": "J. García-Bellido"
    },
    {
      "name": "M. R. S. Hawkins"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/9cgtrxbj1nywz.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.08988",
  "fulltext_url": null,
  "id": "9cgtrxbj1nywz",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.08988",
  "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",
      "astro-ph.GA"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2609.08988",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-08",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/9cgtrxbj1nywz.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:34:18Z",
      "native_id": "2609.08988",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.08988"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.CO",
      "subfield": "cosmology"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "Triaxiality of the Milky Way dark matter halo and the microlensing optical depth towards the Large Magellanic Cloud",
  "title_text": "Triaxiality of the Milky Way dark matter halo and the microlensing optical depth towards the Large Magellanic Cloud",
  "type": "preprint",
  "updated_at": "2026-10-30T12:34:18Z",
  "url": "https://research.draconitic.com/paper/9cgtrxbj1nywz",
  "venue": "arXiv",
  "versions": []
}