GRB 221009A: the B.O.A.T Burst that Shines in Gamma Rays

• Main Authors: Arimoto, M, Baldini, L, Ballet, J, Baring, M. G, Bartolini, C, Gonzalez, J. Becerra, Berenji, B, Bissaldi, E, Blandford, R. D, Bonino, R, Bruel, P, Buson, S, Caputo, R, Cavazzuti, E, Cheung, C. C, Cibrario, N, Ciprini, S, Orestano, P. Cristarella, Crnogorcevic, M, Cutini, S, D'Ammando, F, De Gaetano, S, Di Lalla, N, Ferrara, E. C, Franckowiak, A, Funk, S, Fusco, P, Galanti, G, Gargano, F, Gasbarra, C, Giacchino, F, Giglietto, N, Giliberti, M, Gill, R, Giordano, F, Giroletti, M, Granot, J, Guiriec, S, Gustafsson, M, Hays, E, Horan, D, Laviron, A, Li, J, Liodakis, I, Longo, F, Loparco, F, Lorusso, L, Lovellette, M. N, Malyshev, D, McEnery, J. E, Mirabal, N, Mitthumsiri, W, Mizuno, T, Monti-Guarnieri, P, Monzani, M. E, Morishita, T, Niwa, R, Omodei, N, Orienti, M, Orlando, E, Panzarini, G, Persic, M, Petrosian, V, Pillera, R, Piron, F, Porter, T. A, Principe, G, Racusin, J. L, Rainò, S, Razzano, M, Razzaque, S, Reimer, A, Reimer, O, Ryde, F, Sánchez-Conde, M, Serini, D, Sgrò, C, Sharma, V, Siskind, E. J, Spinelli, P, Suson, D. J, Tajima, H, Thayer, J. B, Torres, D. F, Zaharijas, G, Lesage, S, Briggs, M. S, Burns, E, Bala, S, Bhat, P. N, Dalessi, S, de Barra, C, Gibby, M, Hamburg, R, Kocevski, D, Mailyan, B, Malacaria, C, McBreen, S, Veres, P, Wood, J
• Format: Journal Article
• Language: English
• Published: 06.09.2024
• Subjects: Physics - High Energy Astrophysical Phenomena
• DOI: 10.48550/arxiv.2409.04580

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 (BTI) of 64 seconds 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 seconds 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.