GRB 221009A: The B.O.A.T. Burst that Shines in Gamma Rays
M Axelsson, M Ajello, M. Arimoto, L Baldini, J. Ballet, Matthew G. Baring, C. Bartolini, D. Bastieri, J. Becerra González, R. Bellazzini, B Berenji, E. Bissaldi, R. D. Blandford, R. Bonino, P. Bruel, S Buson, R. A. Cameron, R. Caputo, P. A. Caraveo, E. Cavazzuti, C. C. Cheung, G. Chiaro, Nicolò Cibrario, S. Ciprini, G. Cozzolongo, P. Cristarella Orestano, M. Crnogorčević, A. Cuoco, S. Cutini, F. D’Ammando, S. De Gaetano, N Di Lalla, A. Dinesh, R. Di Tria, L. Di Venere, A. Domínguez, S. Fegan, E. C. Ferrara, A. Fiori, A. Franckowiak, Y Fukazawa, S. Funk, P. Fusco, Giorgio Galanti, F. Gargano, C. Gasbarra, S. Germani, Federica Giacchino, N. Giglietto, M. Giliberti, Ramandeep Gill, F. Giordano, M. Giroletti, J Granot, D. Green, I. A. Grenier, S. Guiriec, M. Gustafsson, M. Hashizume, E. Hays, J. W. Hewitt, D. Horan, T. Kayanoki, M. Kuss, Adrien Laviron, J. Li, I Liodakis, F. Longo, F. Loparco, Leonarda Lorusso, B. Lott, M. N. Lovellette, P. Lubrano, S. Maldera, D. Malyshev, Alberto Manfreda, G. Martí-Devesa, Riccardo Martinelli, Israel Martinez Castellanos, M. N. Mazziotta, J.E Mcenery, I. Mereu, M. Meyer, P. F. Michelson, N. Mirabal, W. Mitthumsiri, T. Mizuno, P. Monti-Guarnieri, M. E. Monzani, T. Morishita, A Morselli, I. V. Moskalenko, Michela Negro, Ryusei Niwa, N Omodei, M. Orienti, E Orlando, D. Paneque, G. Panzarini, M. Persic
Abstract
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.