The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151

Malesani, DB; Levan, AJ; Izzo, L; de Ugarte Postigo, A; Ghirlanda, G; Heintz, KE; Kann, DA; Lamb, GP; Palmerio, J; Salafia, OS; Salvaterra, R; Tanvir, NR; Agüí Fernández, JF; Campana, S; Chrimes, AA; D’Avanzo, P; D’Elia, V; Della Valle, M; De Pasquale, M; Fynbo, JPU; Gaspari, N; Gompertz, BP; Hartmann, DH; Hjorth, J; Jakobsson, P; Palazzi, E; Pian, E; Pugliese, G; Ravasio, ME; Rossi, A; Saccardi, A; Schady, P; Schneider, B; Sollerman, J; Starling, RLC; Thöne, CC; van der Horst, AJ; Vergani, SD; Watson, D; Wiersema, K; Xu, D; Zafar, T; Zheng, SY

The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151

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Malesani, DB ORCID: 0000-0002-7517-326X, Levan, AJ ORCID: 0000-0001-7821-9369, Izzo, L ORCID: 0000-0001-9695-8472, de Ugarte Postigo, A ORCID: 0000-0001-7717-5085, Ghirlanda, G, Heintz, KE, Kann, DA, Lamb, GP ORCID: 0000-0001-5169-4143, Palmerio, J ORCID: 0000-0002-9408-1563, Salafia, OS ORCID: 0000-0003-4924-7322, Salvaterra, R, Tanvir, NR, Agüí Fernández, JF ORCID: 0000-0001-6991-7616, Campana, S ORCID: 0000-0001-6278-1576, Chrimes, AA ORCID: 0000-0001-9842-6808, D’Avanzo, P, D’Elia, V, Della Valle, M, De Pasquale, M, Fynbo, JPU et al (2025) The brightest GRB ever detected: GRB 221009A as a highly luminous event at z = 0.151. Astronomy & Astrophysics, 701. pp. 1-9. ISSN 0004-6361

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Publisher URL: https://doi.org/10.1051/0004-6361/202346146

Abstract

Context. The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons, thus effective probes of the distant Universe. The most luminous bursts are typically detected at moderate and high redshift, where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as those at TeV energies.

Aims. Here we present a spectroscopic redshift measurement for the exceptional GRB 221009A, the brightest GRB observed to date, with emission extending well into the TeV regime.

Methods. We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-infrared (NIR) spectroscopy of the burst afterglow 0.5 days after the explosion.

Results. The spectra exhibit both absorption and emission lines from material in a host galaxy at zGRB = 0.15095 ± 0.00005. Thus, GRB 221009A was a relatively nearby burst with a luminosity distance of dL = 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of long GRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, Eiso > 5 × 1054 erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also most energetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every few decades and once per millennium.

Item Type:	Article
Uncontrolled Keywords:	5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	EDP Sciences
Date of acceptance:	9 June 2024
Date of first compliant Open Access:	17 October 2025
Date Deposited:	17 Oct 2025 08:55
Last Modified:	17 Oct 2025 09:15
DOI or ID number:	10.1051/0004-6361/202346146
URI:	https://researchonline.ljmu.ac.uk/id/eprint/27361

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