{
  "abstract": "We analyze the prompt emission of the exceptionally bright GRB 220426A observed by {\\it Fermi}/GBM, whose time-resolved spectra are among the narrowest measured in any GRB. Here we show that observations during the first $\\sim 5$~s are consistent with the signal being emitted while the jet was still in the initial radiation-dominated acceleration phase. The time-resolved spectra allow the effective launch radius, $r_0$, to be inferred with unusual precision. We find $r_0 \\sim \\mathrm{few} \\times 10^{10}\\,\\mathrm{cm}$, increasing linearly with time. These findings match the theoretical expectation for the recollimation shock, implying that this GRB shows the first clear evidence for the existence and evolution of a recollimation shock. Using this interpretation, the linear increase observed is sustained over a period longer than expected from a pure jet breakout. Therefore, the jet collimation must have persisted even after breakout. We suggest that the collimation was maintained by a finite, dense, wind-like circumburst medium, which would reproduce the observed behavior of the prompt emission. We conclude that late-stage progenitor mass-loss can shape the earliest prompt emission and that the photospheric emission provides a way to probe both the jet collimation and the innermost region of the circumburst medium (CBM) surrounding the progenitor, independently of the constraints from interacting supernovae.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2608.10841",
  "author_total": 2,
  "authors": [
    {
      "name": "Felix Ryde"
    },
    {
      "name": "Asaf Pe'er"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/99qpk96kzedtm.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2608.10841",
  "fulltext_url": null,
  "id": "99qpk96kzedtm",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2608.10841",
  "language": null,
  "licence": "CC-BY-NC-ND-4.0",
  "licence_label": "CC BY-NC-ND 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by-nc-nd/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.HE"
    ],
    [
      "openalex",
      "3103|Gamma-ray bursts and supernovae"
    ]
  ],
  "openalex": "W7202272915",
  "pdf_url": "https://arxiv.org/pdf/2608.10841",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-24",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/99qpk96kzedtm.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:24:11Z",
      "native_id": "2608.10841",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2608.10841"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "title": "Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A",
  "title_text": "Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A",
  "type": "preprint",
  "updated_at": "2026-10-30T12:24:11Z",
  "url": "https://research.draconitic.com/paper/99qpk96kzedtm",
  "venue": "arXiv",
  "versions": []
}