{
  "abstract": "We present X-ray, optical, and radio follow-up observations of EP250304a, an extragalactic fast X-ray transient (EFXT) discovered by the Einstein Probe. Its X-ray light curve exhibits two broad pulses with comparable peak fluxes within the first $\\sim$1~ks, a feature rarely seen among low-luminosity gamma-ray bursts or EFXTs. Optical follow-up observations were carried out using the Korea Microlensing Telescope Network, the Thai Robotic Telescope, the Las Cumbres Observatory 1~m global network, the Gemini Multi-Object Spectrograph on Gemini south telescope, and the Global Supernova Network. The fast-cooling phase (within 3 days) of optical data can be well fitted by a shocked cocoon model. However, during the supernova phase (SN 2025fhm, from 3 to 88 days), the late-time light curve cannot be explained solely by radioactive $^{56}$Ni decay, as demonstrated by a grid of simulations using the one-dimensional Lagrangian radiation hydrodynamics code SNEC, which reveals a significant energy excess at late epochs. To account for this excess, a central engine like a rapidly spinning, highly magnetized neutron star is needed to provide additional energy injection. This model yields a best-fit spin period of $\\sim$12.60~ms and magnetic field strength of $\\sim 3.52\\times10^{15} \\rm G$, and it successfully explains both the late-time bolometric light curve and the early X-ray pulse structures. Our results indicate that EP250304a/SN 2025fhm is likely powered by a central magnetar rather than by radioactive decay alone, offering new insights into the energy budget and physical origin of EFXTs and their associated supernovae.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.08356",
  "author_total": 35,
  "authors": [
    {
      "name": "Cui-Ying Song"
    },
    {
      "name": "Nan Jiang"
    },
    {
      "name": "Xiaofeng Wang"
    },
    {
      "name": "Yi-Han Iris Yin"
    },
    {
      "name": "Lingzhi Wang"
    },
    {
      "name": "Wenxiong Li"
    },
    {
      "name": "Shengyu Yan"
    },
    {
      "name": "Dae-Sik Moon"
    },
    {
      "name": "Tao An"
    },
    {
      "name": "Aleksandar Cikota"
    },
    {
      "name": "Samaporn Tinyanont"
    },
    {
      "name": "Liang-Duan Liu"
    },
    {
      "name": "Cui-Yuan Dai"
    },
    {
      "name": "Christopher D. Matzner"
    },
    {
      "name": "Bin-Bin Zhang"
    },
    {
      "name": "Lixin Yu"
    },
    {
      "name": "Qinyu Wu"
    },
    {
      "name": "Hong Soo Park"
    },
    {
      "name": "Sang Chul Kim"
    },
    {
      "name": "Youngdae Lee"
    },
    {
      "name": "Yu-Hao Zhang"
    },
    {
      "name": "Haowei Peng"
    },
    {
      "name": "Franz E. Bauer"
    },
    {
      "name": "Joseph R. Farah"
    },
    {
      "name": "Moira Andrews"
    },
    {
      "name": "Kathryn Wynn"
    },
    {
      "name": "Yuan Qi Ni"
    },
    {
      "name": "D. Andrew Howell"
    },
    {
      "name": "Curtis McCully"
    },
    {
      "name": "Ning-Chen Sun"
    },
    {
      "name": "Danfeng Xiang"
    },
    {
      "name": "Yuan Liu"
    },
    {
      "name": "Wenxin Wang"
    },
    {
      "name": "Yijia Zhang"
    },
    {
      "name": "Wei Chen"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/01v64ep97gqw0.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.08356",
  "fulltext_url": null,
  "id": "01v64ep97gqw0",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.08356",
  "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"
    ],
    [
      "arxiv",
      "astro-ph.SR"
    ],
    [
      "openalex",
      "3103|Gamma-ray bursts and supernovae"
    ]
  ],
  "openalex": "W7212038601",
  "pdf_url": "https://arxiv.org/pdf/2609.08356",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-08",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/01v64ep97gqw0.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:19:49Z",
      "native_id": "2609.08356",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.08356"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "subfield": "stellar"
    }
  ],
  "title": "SN 2025fhm: A central-engine powered Ic-BL supernova associated with X-ray transient EP250304a",
  "title_text": "SN 2025fhm: A central-engine powered Ic-BL supernova associated with X-ray transient EP250304a",
  "type": "preprint",
  "updated_at": "2026-10-30T12:19:49Z",
  "url": "https://research.draconitic.com/paper/01v64ep97gqw0",
  "venue": "arXiv",
  "versions": []
}