{
  "abstract": "Neutron star mergers (NSMs) are confirmed gravitational wave sources. Identifying an early electromagnetic counterpart for these events is therefore crucial for rapid localization and multimessenger follow-up. However, the associated Gamma-Ray Bursts (GRBs) are highly collimated and are therefore easily missed by off-axis observers. An early, less beamed counterpart is essential for identifying the majority of mergers. In this Letter, we investigate the cooling emission from jet-driven cocoons produced by short gamma-ray burst jets propagating through merger ejecta. We perform hydrodynamic simulations and radiative post-processing to calculate the early X-ray emission over a wide range of viewing angles. We find that the mildly relativistic cocoon produces bright soft X-ray transients for off-axis observers, with luminosities of $10^{46-48}{\\rm erg\\,s^{-1}}$ and durations of a few to ten seconds. The X-ray spectra are quasi-thermal with characteristic temperatures of $0.1$--$1\\,{\\rm keV}$. For observers at $θ_{\\rm v}=10^{\\circ}$, the cocoon emission is detectable by Einstein Probe (EP) out to $z\\simeq 0.3$. For nearby events like GW170817, it remains detectable up to $θ_{\\rm v}\\simeq 45^{\\circ}$. The predicted detection rate for EP is $0.5\\,{\\rm yr^{-1}}$ in the canonical model. In future multimessenger campaigns, rapid UV/optical/IR follow-up of such X-ray triggers can subsequently identify the associated kilonova and jet afterglow emission, which will confirm the origin.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": "Journal reference: ApJL 1010 L7 (2026)",
  "arxiv": "2609.14508",
  "author_total": 2,
  "authors": [
    {
      "name": "Jian-He Zheng"
    },
    {
      "name": "Wenbin Lu"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2dvwffbv9qxac.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.14508",
  "fulltext_url": null,
  "id": "2dvwffbv9qxac",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.14508",
  "language": null,
  "licence": "CC-BY-NC-SA-4.0",
  "licence_label": "CC BY-NC-SA 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by-nc-sa/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.HE"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2609.14508",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-02",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/2dvwffbv9qxac.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:29:06Z",
      "native_id": "2609.14508",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.14508"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-ray Emission from Short Gamma-Ray Burst Jets",
  "title_text": "First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-ray Emission from Short Gamma-Ray Burst Jets",
  "type": "preprint",
  "updated_at": "2026-10-30T12:29:06Z",
  "url": "https://research.draconitic.com/paper/2dvwffbv9qxac",
  "venue": "arXiv",
  "versions": [
    {
      "doi": "10.3847/2041-8213/aeac25",
      "pid": "fcmw1w6cdqaew",
      "relation": "published-as",
      "title": "First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-Ray Emission from Short Gamma-Ray Burst Jets",
      "venue": "The Astrophysical Journal Letters"
    }
  ]
}