{
  "abstract": "A0538-66 is a peculiar Be/X-ray binary in the Large Magellanic Cloud, hosting the fastest spinning neutron star of its class (~69ms) and showing the brightest optical bursts of any system of this type. We report four XMM-Newton observations obtained near periastron in 2025, together with MAXI and REM monitoring, and a reanalysis of archival RXTE data. On 30 September 2025 the source reached L_X~1.5E39 erg/s (0.2-10 keV), the highest level since 1980, and in February, May, and August 2026, MAXI detected similarly bright outbursts. During the XMM-Newton observation taken on 30 September 2025, we detected the pulsations, measuring P_spin=69.25878(2) ms and dP_spin/dt=6.58(9)E-10 s/s; the pulsed fraction was low (~3% in 1-10 keV), increased with energy, and the soft and hard pulse profiles were shifted by half a cycle. A new detection in RXTE data of 3 December 1998 gives a faster spin period of P_spin=68.88813(28) ms. The XMM-Newton/RGS spectra show emission lines from photoionised plasma at rest and blueshifted absorption lines that imply an outflow at ~0.12c. This indicates a highly super-Eddington accretion rate and an intrinsic L_X~1.5E39 erg/s, and it makes A0538-66 a transient ultra-luminous X-ray pulsar, a class of which only a few objects are known so far. Since 2023 the optical bursts observed by REM have become longer and brighter, resembling those of the 1980s, but most of them lack a luminous X-ray counterpart (L_X>1E37 erg/s). We interpret these results in terms of a radiation pressure dominated accretion disc whose geometrically thick inner regions intermittently screen the innermost emitting regions.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.17982",
  "author_total": 11,
  "authors": [
    {
      "name": "Lorenzo Ducci"
    },
    {
      "name": "Sandro Mereghetti"
    },
    {
      "name": "Ciro Pinto"
    },
    {
      "name": "Matteo Bachetti"
    },
    {
      "name": "Motoki Nakajima"
    },
    {
      "name": "Rahul Sharma"
    },
    {
      "name": "Stefano Covino"
    },
    {
      "name": "Michela Rigoselli"
    },
    {
      "name": "Caterina Tresoldi"
    },
    {
      "name": "Tatehiro Mihara"
    },
    {
      "name": "Mutsumi Sugizaki"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/69rwfq4bb8z2f.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.17982",
  "fulltext_url": null,
  "id": "69rwfq4bb8z2f",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.17982",
  "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.HE"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2610.17982",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-13",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/69rwfq4bb8z2f.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:29:11Z",
      "native_id": "2610.17982",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.17982"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "A0538-66: A transient ultraluminous X-ray source with a relativistic outflow, 69ms pulsations, and anomalous optical activity",
  "title_text": "A0538-66: A transient ultraluminous X-ray source with a relativistic outflow, 69ms pulsations, and anomalous optical activity",
  "type": "preprint",
  "updated_at": "2026-10-30T12:29:11Z",
  "url": "https://research.draconitic.com/paper/69rwfq4bb8z2f",
  "venue": "arXiv",
  "versions": []
}