{
  "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 postprocessing 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 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 keV. For observers at θ v = 10 ∘ , the cocoon emission is detectable by the Einstein Probe (EP) out to z ≃ 0.3. For nearby events like GW170817, it remains detectable up to θ v ≃ 45 ∘ . The predicted detection rate for EP is 0.5 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 can confirm their origin.",
  "abstract_shown": true,
  "affiliations": [
    "Nanjing University",
    "University of California"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 2,
  "authors": [
    {
      "affs": [
        0,
        1
      ],
      "name": "Jian-He Zheng",
      "orcid": "0000-0001-5751-633X"
    },
    {
      "affs": [
        1
      ],
      "name": "Wenbin Lu",
      "orcid": "0000-0002-1568-7461"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/fcmw1w6cdqaew.bib",
  "cited_by": 0,
  "doi": "10.3847/2041-8213/aeac25",
  "fulltext_url": null,
  "id": "fcmw1w6cdqaew",
  "issns": [
    "2041-8205",
    "2041-8213"
  ],
  "landing_url": "https://iopscience.iop.org/article/10.3847/2041-8213/aeac25",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "3103|Gamma-ray bursts and supernovae"
    ]
  ],
  "openalex": "W7213359995",
  "pdf_url": "https://iopscience.iop.org/article/10.3847/2041-8213/aeac25/pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-06",
  "publisher": "American Astronomical Society",
  "ris": "https://research.draconitic.com/paper/fcmw1w6cdqaew.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:13Z",
      "native_id": "10.3847/2041-8213/aeac25",
      "source": "crossref",
      "url": "https://iopscience.iop.org/article/10.3847/2041-8213/aeac25"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "openalex:3103|Gamma-ray bursts and supernovae",
      "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": "peer-reviewed",
  "updated_at": "2026-10-30T12:18:13Z",
  "url": "https://research.draconitic.com/paper/fcmw1w6cdqaew",
  "venue": "The Astrophysical Journal Letters",
  "versions": [
    {
      "doi": "10.48550/arxiv.2609.14508",
      "pid": "2dvwffbv9qxac",
      "relation": "preprint",
      "title": "First Light of Neutron Star Mergers: Off-axis Cocoon Cooling X-ray Emission from Short Gamma-Ray Burst Jets",
      "venue": "arXiv"
    }
  ]
}