GRB 221009A: The B.O.A.T. Burst that Shines in Gamma Rays
Authors: Axelsson M. , Ajello M. , Arimoto M. , Baldini L. , Ballet J. , Baring M. G. , Bartolini C. , Bastieri D. , Becerra Gonzalez J. , Bellazzini R. , Berenji B. , Bissaldi E. , Blandford R. D. , Bonino R. , Bruel P. , Buson S. , Cameron R. A. , Caputo R. , Caraveo P. A. , Cavazzuti E. , Cheung C. C. , Chiaro G. , Cibrario N. , Ciprini S. , Cozzolongo G. , Cristarella Orestano P. , Crnogorcevic M. , Cuoco A. , Cutini S. , D’Ammando F. , De Gaetano S. , Di Lalla N. , Dinesh A. , Di Tria R. , Di Venere L. , Domínguez A. , Fegan S. J. , Ferrara E. C. , Fiori A. , Franckowiak A. , Fukazawa Y. , Funk S. , Fusco P. , Galanti G. , Gargano F. , Gasbarra C. , Germani S. , Giacchino F. , Giglietto N. , Giliberti M. , Gill R. , Giordano F. , Giroletti M. , Granot J. , Green D. , Grenier I. A. , Guiriec S. , Gustafsson M. , Hashizume M. , Hays E. , Hewitt J. W. , Horan D. , Kayanoki T. , Kuss M. , Laviron A. , Li J. , Liodakis I. , Longo F. , Loparco F. , Lorusso L. , Lott B. , Lovellette M. N. , Lubrano P. , Maldera S. , Malyshev D. , Manfreda A. , Martí-Devesa G. , Martinelli R. , Martinez Castellanos I. , Mazziotta M. N. , McEnery J. E. , Mereu I. , Meyer M. , Michelson P. F. , Mirabal N. , Mitthumsiri W. , Mizuno T. , Monti-Guarnieri P. , Monzani M. E. , Morishita T. , Morselli A. , Moskalenko I. V. , Negro M. , Niwa R. , Omodei N. , Orienti M. , Orlando E. , Paneque D. , Panzarini G. , Persic M. , Pesce-Rollins M. , Petrosian V. , Pillera R. , Piron F. , Porter T. A. , Principe G. , Racusin J. L. , Rainò S. , Rando R. , Rani B. , Razzano M. , Razzaque S. , Reimer A. , Reimer O. , Ryde F. , Sánchez-Conde M. , Saz Parkinson P. M. , Serini D. , Sgrò C. , Sharma V. , Siskind E. J. , Spandre G. , Spinelli P. , Suson D. J. , Tajima H. , Tak D. , Thayer J. B. , Torres D. F. , Valverde J. , Zaharijas G. ,   (Fermi LAT collaboration), Lesage S. , Briggs M. S. , Burns E. , Bala S. , Bhat P. N. , Cleveland W. H. , Dalessi S. , de Barra C. , Gibby M. , Giles M. M. , Hamburg R. , Hristov B. A. , Kocevski D. , Hui C. M. , Mailyan B. , Malacaria C. , McBreen S. , Poolakkil S. , Roberts O. J. , Scotton L. , Veres P. , von Kienlin A. , Wilson-Hodge C. A. , Wood J. ,   (Fermi GBM collaboration)
Published: February 2025

Abstract
We present a complete analysis of Fermi Large Area Telescope (LAT) data of GRB 221009A, the brightest gamma-ray burst (GRB) ever detected. The burst emission above 30 MeV detected by the LAT preceded, by 1 s, the low-energy (<10 MeV) pulse that triggered the Fermi Gamma-Ray Burst Monitor (GBM), as has been observed in other GRBs. The prompt phase of GRB 221009A lasted a few hundred seconds. It was so bright that we identify a bad time interval of 64 s caused by the extremely high flux of hard X-rays and soft gamma rays, during which the event reconstruction efficiency was poor and the dead time fraction quite high. The late-time emission decayed as a power law, but the extrapolation of the late-time emission during the first 450 s suggests that the afterglow started during the prompt emission. We also found that high-energy events observed by the LAT are incompatible with synchrotron origin, and, during the prompt emission, are more likely related to an extra component identified as synchrotron self-Compton (SSC). A remarkable 400 GeV photon, detected by the LAT 33 ks after the GBM trigger and directionally consistent with the location of GRB 221009A, is hard to explain as a product of SSC or TeV electromagnetic cascades, and the process responsible for its origin is uncertain. Because of its proximity and energetic nature, GRB 221009A is an extremely rare event.