{
  "abstract": "Neutron star-bearing compact-object mergers can create a kilonova, a transient powered by the radioactive decay of material synthesized via rapid neutron capture ($r$-process). During the merger, a fraction of the ejecta is launched at high speeds ($\\gtrsim$ 0.4c) that can lead to neutrons evading capture onto seed nuclei, resulting in free neutrons. The free neutrons then decay and inject additional energy that power early-time ($\\lesssim$1 day) emission. Here, using \\texttt{Sedona}, we present a grid of the first non-local thermodynamic equilibrium and frequency-dependent opacity radiative transfer simulations to predict early X-ray/UV/optical emission from free neutron decay that span the expected theoretical range of free neutron mass, mixing with $r$-process material, and extent of the high-velocity ejecta tail. The emission can be characterized by an SED that rapidly shifts from X-rays of $\\sim\\rm{few} \\times10^{41}-10^{42}$ erg s$^{-1}$ in the first $\\sim$ minutes to a far-UV and near-UV peak on the timescale of $\\sim$ minutes to hours, followed by enhanced optical and IR emission for sufficiently large free neutron masses. The properties of the free neutron ejecta are most distinguishable, in principle, at extreme-UV and far-UV wavelengths, with SED peak flux and wavelength determined by the maximum velocity of the ejecta and the mixing. In the far-UV and near-UV, free neutron ejecta masses as small as 10$^{-7}$ M$_\\odot$ exhibit a unique bump compared to neutron-free models, demonstrating the importance of upcoming UV missions like UVEX and ULTRASAT to constraining the nucleosynthetic environment of neutron star mergers.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.09311",
  "author_total": 5,
  "authors": [
    {
      "name": "Daniel Brethauer"
    },
    {
      "name": "Daniel Kasen"
    },
    {
      "name": "Smaranika Banerjee"
    },
    {
      "name": "Raffaella Margutti"
    },
    {
      "name": "Ryan Chornock"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2xwshhnt6kbyq.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.09311",
  "fulltext_url": null,
  "id": "2xwshhnt6kbyq",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.09311",
  "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",
      "3103|Gamma-ray bursts and supernovae"
    ]
  ],
  "openalex": "W7212119752",
  "pdf_url": "https://arxiv.org/pdf/2609.09311",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-08",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/2xwshhnt6kbyq.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:20:12Z",
      "native_id": "2609.09311",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.09311"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "Free Neutron Decay in Kilonova Ejecta: X-ray/UV Flashes with Non-Thermal Effects",
  "title_text": "Free Neutron Decay in Kilonova Ejecta: X-ray/UV Flashes with Non-Thermal Effects",
  "type": "preprint",
  "updated_at": "2026-10-30T12:20:12Z",
  "url": "https://research.draconitic.com/paper/2xwshhnt6kbyq",
  "venue": "arXiv",
  "versions": []
}