TITLE: GCN CIRCULAR NUMBER: 32642 SUBJECT: GRB 221009A: Fermi GBM observation DATE: 22/10/10 04:04:41 GMT FROM: Stephen Lesage at Fermi-GBM Team S. Lesage (UAH), P. Veres (UAH), O.J. Roberts (USRA), E. Burns (LSU), and E. Bissaldi (Politecnico and INFN Bari) report on behalf of the Fermi GBM Team: "At 13:16:59.99 UT on 09 October 2022, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 221009A (trigger 687014224/221009553) which was also detected by Swift-BAT (S. Dichiara, et al. 2022, GCN 32632; J. A. Kennea, et al. 2022, GCN 32635), Fermi-LAT (E. Bissaldi et al. 2022, GCN 32637), INTEGRAL (SPI-ACS), Konus-Wind, and triangulated by IPN (D. Svinkin et al. 2022, GCN 32641). The GBM on-ground location (GCN 32636) is consistent with the Swift-BAT and Fermi-LAT locations and the IPN localization. The angle from the Fermi LAT boresight is 73 degrees. The GBM light curve consists of two emission episodes, a single isolated peak followed by a longer, extremely bright, multi-pulsed emission episode with a duration (T90) of about 327 s (10-1000 keV). The time-averaged spectrum for the first emission episode from T0-0.0 to T0+43.4 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.70 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 375 +/- 87 keV. The event fluence (10-1000 keV) in this time interval is (2.12 +/- 0.05)E-05 erg/cm^2. Due to the brightness of the second emission episode, the likelihood of pulse pile-up and other systematic effects are very high and no single spectral model provides an adequate fit in this preliminary analysis. The event fluence (10-1000 keV) from T0+175 to T0+1458 s is on the order of (2.912 +/- 0.001)E-02 erg/cm^2. The 1.024 sec peak photon flux measured starting from T0+238.8 s in the 10-1000 keV band is on the order of 2385 +/- 3 ph/s/cm^2, making this the most intense and fluent GRB detected by Fermi GBM. Further analysis is being performed. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"