GRB 221009A: Fermi-LAT refined analysis

We report updated observations of GRB 221009A which was detected by Swift (Kennea et al. GCN #32635), Fermi-GBM (Veres et al. GCN #32636, Lesage et al. GCN #32642), Fermi-LAT (Bissaldi et al. GCN #32637), and the IPN (Svinkin et al. GCN #32641). GRB 221009A triggered Fermi-GBM on October 10, 2022, at 13:16:59.99 UT (trigger 687014224/221009553), about 1 hour earlier with respect to the Swift trigger, which was reported as a new bright hard X-ray and optical transient and tentatively classified as Swift J1913.1+1946 (Dichiara et al., GCN 32632). Prompt GCN notices from Fermi-GBM were not distributed due to problems with the real-time downlink from TDRS, therefore no automatic Fermi-LAT GRB pipelines were triggered by the GBM event. Using LAT events with E>100 MeV between T0+200 s and T0+800 s, we find a LAT localization of RA = 288.282, Dec = 19.495, with a 90% containment radius of 0.027 degrees (statistical only). The LAT lightcurve shows a bright structured emission episode which is temporally coincident with the GBM main emission episode starting at T0+200s. The 100 MeV - 1 GeV photon flux in the time interval 200-800 s after the GBM trigger is (6.2 +/- 0.4)E-03 ph/cm2/s. The estimated photon index above 100 MeV is -1.87 +/- 0.04. From a preliminary analysis, the LAT emission is extending for about 25ks post GBM trigger. The highest-energy photon is 99.3 GeV (with a probability of 99.2%) which is observed 240 seconds after the GBM trigger. This represents the highest GRB photon energy ever detected by Fermi-LAT (the previous record holder being a 95 GeV event from GRB 130427A). The Fermi-LAT point of contact for this burst is Elisabetta Bissaldi (elisabetta.bissaldi@ba.infn.it). The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.