TITLE: GCN CIRCULAR NUMBER: 32866 SUBJECT: GRB 221027A: Fermi GBM detection DATE: 22/10/27 14:45:08 GMT FROM: Oliver J Roberts at USRA/NASA O.J. Roberts (USRA/NASA-MSFC) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 04:09:34.35 UT on 27 October 2022, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 221027A (trigger 688536579 / 221027173), which was also detected by Fermi-LAT (M. Arimoto et al. 2022, GCN 32862). The Fermi GBM Final Real-time Localization (GCN 32861) is consistent with the LAT position. The angle from the Fermi LAT boresight at the GBM trigger time is 51 degrees. The GBM light curve consists of a bright pulse with a duration (T90) of about 7 s (50-300 keV). The time-averaged spectrum from T0-1.5s to T0+9.7 s is best fit by a Band function with Epeak = 341 +/- 37 keV, alpha = -0.75 +/- 0.06, and beta = -2.09 +/- 0.14. The event fluence (10-1000 keV) in this time interval is (9.2 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1.2 s in the 10-1000 keV band is 8.5 +/- 0.3 ph/s/cm^2. A power law function with an exponential high-energy cutoff fits the spectrum equally well. The power law index is -0.81 +/- 0.04 and the cutoff energy, parameterized as Epeak, is 411 +/- 29 keV 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/"