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Peer-reviewedCC BY 4.0Monthly Notices of the Royal Astronomical Society

JWST’s Dusty Blue BOAT - GRB 221009A

  • Nguyen M Khang1
  • Gavin P Lamb1
  • Helena-M S Grabham1
  • Conor M B Omand1
  • Hamid Hamidani2,3
  • Andrew J Levan4,5
  • Nial R Tanvir6
  • Valerio D’Elia7
  • Luca Izzo8,9
9 affiliations
  1. 1Astrophysics Research Institute, Liverpool John Moores University , IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF ,
  2. 2School of Systems Information Science, Future University Hakodate , Hakodate, Hokkaido 041-8655 ,
  3. 3Astronomical Institute, Graduate School of Science , Tohoku University, Sendai 980-8578 ,
  4. 4Radboud University Department of Astrophysics/IMAPP, , 6525 AJ Nijmegen ,
  5. 5University of Warwick Department of Physics, , Coventry, CV4 7AL ,
  6. 6School of Physics and Astronomy, University of Leicester, University Road, Leicester , LE1 7RH ,
  7. 7Space Science Data Center (SSDC) - Agenzia Spaziale Italiana (ASI) , I-00133 Roma ,
  8. 8DARK, Niels Bohr Institute, University of Copenhagen , Jagtvej 155A, 2200 Copenhagen ,
  9. 9INAF, Osservatorio Astronomico di Capodimonte , Salita Moiariello 16, I-80121 Naples ,
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Abstract

GRB 221009A, the Brightest Of All Time (BOAT), presents a challenge for afterglow modelling due to its low Galactic latitude and consequent high line-of-sight extinction. This has led to a wide range of conflicting values for the optical spectral index and dust extinction in the literature. We present a re-analysis of the afterglow spectra, using VLT X-Shooter data at 0.5, 4, and 10.5 days, and JWST NIRSpec/MIRI data at 13.3 days post-burst. We fit the data with single and smoothly broken power-law models and perform a joint fit with a double smoothly broken power-law (DSBPL) across all epochs. Our analysis reveals a strong degeneracy between the assumed extinction and the inferred intrinsic spectral index, particularly in the optical, explaining the diversity of previous results. The joint DSBPL fit yields a total line-of-sight extinction of AV = 4.40 ± 0.01 and a blue continuum, with an intrinsic spectral index of β = 0.447 ± 0.001. Although marginally preferred by the spectral fits, a wind medium can be rejected by the temporal evolution of the afterglow light curve. The fit spectral index and temporal decline are only consistent with a uniform density medium if an early jet break at ∼0.5-1.0 days is invoked. Our results imply a hard electron distribution index of p = 1.89 < 2, challenging standard particle acceleration models and suggesting a narrow, energetic jet core dominates the early optical-to-X-ray emission.

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Cite

BibTeXRIS
BibTeX
@article{khang2026jwsts,
  title = {{JWST’s Dusty Blue BOAT - GRB 221009A}},
  author = {Nguyen M Khang and Gavin P Lamb and Helena-M S Grabham and Conor M B Omand and Hamid Hamidani and Andrew J Levan and Nial R Tanvir and Valerio D’Elia and Luca Izzo},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year = {2026},
  month = oct,
  doi = {10.1093/mnras/stag2053},
  url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2053/9005956},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - JWST’s Dusty Blue BOAT - GRB 221009A
AU  - Nguyen M Khang
AU  - Gavin P Lamb
AU  - Helena-M S Grabham
AU  - Conor M B Omand
AU  - Hamid Hamidani
AU  - Andrew J Levan
AU  - Nial R Tanvir
AU  - Valerio D’Elia
AU  - Luca Izzo
PY  - 2026
DA  - 2026/10/31
JO  - Monthly Notices of the Royal Astronomical Society
DO  - 10.1093/mnras/stag2053
UR  - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2053/9005956
AB  - GRB 221009A, the Brightest Of All Time (BOAT), presents a challenge for afterglow modelling due to its low Galactic latitude and consequent high line-of-sight extinction. This has led to a wide range of conflicting values for the optical spectral index and dust extinction in the literature. We present a re-analysis of the afterglow spectra, using VLT X-Shooter data at 0.5, 4, and 10.5 days, and JWST NIRSpec/MIRI data at 13.3 days post-burst. We fit the data with single and smoothly broken power-law models and perform a joint fit with a double smoothly broken power-law (DSBPL) across all epochs. Our analysis reveals a strong degeneracy between the assumed extinction and the inferred intrinsic spectral index, particularly in the optical, explaining the diversity of previous results. The joint DSBPL fit yields a total line-of-sight extinction of AV = 4.40 ± 0.01 and a blue continuum, with an intrinsic spectral index of β = 0.447 ± 0.001. Although marginally preferred by the spectral fits, a wind medium can be rejected by the temporal evolution of the afterglow light curve. The fit spectral index and temporal decline are only consistent with a uniform density medium if an early jet break at ∼0.5-1.0 days is invoked. Our results imply a hard electron distribution index of p = 1.89 < 2, challenging standard particle acceleration models and suggesting a narrow, energetic jet core dominates the early optical-to-X-ray emission.
SN  - 0035-8711
SN  - 1365-2966
ER  -