Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A

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 ∼ 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, r₀, to be inferred with unusual precision. We find r₀ ∼ few × 10¹⁰ 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.

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BibTeX RIS

BibTeX

@misc{ryde2026windconfined,
  title = {{Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A}},
  author = {Felix Ryde and Asaf Pe'er},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2608.10841},
  eprint = {2608.10841},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2608.10841},
  note = {CC BY-NC-ND 4.0},
}

RIS

TY  - UNPB
TI  - Wind-confined jet collimation revealed by the acceleration-phase photosphere of GRB 220426A
AU  - Felix Ryde
AU  - Asaf Pe'er
PY  - 2026
DA  - 2026/09/24
PB  - arXiv
DO  - 10.48550/arxiv.2608.10841
UR  - https://arxiv.org/abs/2608.10841
AB  - 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 ∼ 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 ∼ few × 10^10 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.
ER  -

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