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

GRB 221009A, also known as the B.O.A.T, was observed by tens of space- and ground-based observatories, including both instruments onboard the Fermi telescope, the Large Area Telescope (LAT) and the Gamma-Ray Burst Monitor (GBM). The triggering pulse, detected by Fermi-GBM, was followed by a prompt phase lasting a few hundred seconds, and by an extended emission which was detected by Fermi-LAT for over two days. Here we present the highlights from the LAT analysis of this exceptional event including the effort to recover data during the extreme intense hard X-ray and soft gamma-ray flux, which compromised the LAT data quality for about one minute. Furthermore, we found that the high-energy events detected by the LAT cannot have a Synchrotron origin but, during the prompt emission, they are probably associated with an additional Self Synchrotron Compton (SSC) component, which is also repsonsible with the TeV emission detected by the Large High Altitude Air Shower Observatory (LHAASO). Late time high-energy events are instead harder to explain as products of SSC or TeV electromagnetic cascades, which raises questions regarding their origin. Overall, GRB221009A, stands out compared to other Fermi-LAT GRBs, indicating that it is an exceptionally rare event.