JWST's Dusty Blue BOAT -- GRB 221009A
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.
Related
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- Blue-tilted spectral running and the JWST early galaxy tension
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- Pair echo model of CTAO/LST-1 signal from GRB 221009A
BibTeX
@misc{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},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2605.29022},
eprint = {2605.29022},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.29022},
note = {CC BY 4.0},
}
RIS
TY - UNPB
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 - 2026/11/05
PB - arXiv
DO - 10.48550/arxiv.2605.29022
UR - https://arxiv.org/abs/2605.29022
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 A_V = 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.
ER -