The GeV View of the Brightest GRB Ever Detected

On October 9, 2022, the brightest GRB ever detected, also called the “Brightest Of All Time” (or BOAT), was detected by the Fermi Large Area Telescope (LAT), providing an unprecedented rich dataset that will likely help to understand the emission mechanisms underlying this class of extra-galactic objects. Some of the unique features of GRB221009A include: 1) high-energy (up to 100 MeV) emission observed during the first emission episode (precursor) at the GBM trigger time T0; 2) Several GeV events, including a 100 GeV photon, observed during the brightest emission episode; and 3) a very long-lived high-energy extended emission phase covering tens of kiloseconds, with a peculiar 400 GeV photon observed at almost 10 hours after T0. In this talk I will review all these observations, focusing especially on the efforts that the Fermi LAT collaboration made in order to understand and recover part of the data affected by the extremely high soft gamma-ray pile-up effect during the brightest emission episode at T0+200.