The First JWST Spectrum of a GRB Afterglow: No Bright Supernova in Observations of the Brightest GRB of all Time, GRB 221009A

Levan, A. J. and Lamb, G. P. and Schneider, B. and Hjorth, J. and Zafar, T. and de Ugarte Postigo, A. and Sargent, B. and Mullally, S. E. and Izzo, L. and D’Avanzo, P. and Burns, E. and Fernández, J. F. Agüí and Barclay, T. and Bernardini, M. G. and Bhirombhakdi, K. and Bremer, M. and Brivio, R. and Campana, S. and Chrimes, A. A. and D’Elia, V. and Valle, M. Della and De Pasquale, M. and Ferro, M. and Fong, W. and Fruchter, A. S. and Fynbo, J. P. U. and Gaspari, N. and Gompertz, B. P. and Hartmann, D. H. and Hedges, C. L. and Heintz, K. E. and Hotokezaka, K. and Jakobsson, P. and Kann, D. A. and Kennea, J. A. and Laskar, T. and Le Floc’h, E. and Malesani, D. B. and Melandri, A. and Metzger, B. D. and Oates, S. R. and Pian, E. and Piranomonte, S. and Pugliese, G. and Racusin, J. L. and Rastinejad, J. C. and Ravasio, M. E. and Rossi, A. and Saccardi, A. and Salvaterra, R. and Sbarufatti, B. and Starling, R. L. C. and Tanvir, N. R. and Thöne, C. C. and van der Horst, A. J. and Vergani, S. D. and Watson, D. and Wiersema, K. and Wijers, R. A. M. J. and Xu, Dong (2023) The First JWST Spectrum of a GRB Afterglow: No Bright Supernova in Observations of the Brightest GRB of all Time, GRB 221009A. Astrophysical Journal Letters, 946 (1). ISSN 2041-8205

Abstract

We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/Near Infrared Spectrograph (0.6–5.5 micron) and Mid-Infrared Instrument (5–12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power law, with F ν ∝ ν −β , we obtain β ≈ 0.35, modified by substantial dust extinction with A V = 4.9. This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post-jet-break model, with electron index p < 2, and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/near-IR to X-SHOOTER spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disk-like host galaxy, viewed close to edge-on, that further complicates the isolation of any SN component. The host galaxy appears rather typical among long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.