TITLE: GCN CIRCULAR NUMBER: 30947 SUBJECT: GRB 211018A: Fermi GBM observation DATE: 21/10/19 17:18:39 GMT FROM: Oliver J Roberts at USRA/NASA <oliver.roberts@nasa.gov> O.J Roberts (USRA/NASA-MSFC) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 22:28:13.94 UT on 18 October 2021, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 211018A (trigger 656288898 / 211018936), which was also detected by the Fermi LAT (M. Axelsson et al. 2021, GCN 30943), and GRBAlpha (M. Ohno, GCN 30945). The GBM on-ground location is consistent with the Fermi-LAT position. The angle from the Fermi LAT boresight at the GBM trigger time is 68 degrees. The GBM light curve consists of multiple peaks with a duration (T90) of 123.9 s (50-300 keV). The time-averaged spectrum from T-2.4s to T0+142.0 s is well fit by a power law function with an exponential high-energy cutoff. The power law index is -0.75 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 351.4 +/- 4.9 keV. The Band function also fits the burst equally well, with parameters, alpha= -0.74 +/- 0.01, beta= -3.13 +/- 0.28 and Epeak of 344.6 +/- 6.2 keV. The event fluence (10-1000 keV) in the T90 interval is (1.53347 +/- 0.0003)E-04 erg/cm^2. The 1-sec peak photon flux measured starting from T0+134.5 s in the 10-1000 keV band is 15.6 +/- 0.3 ph/s/cm^2. We note that owing to the large total fluence of this burst, the 1-sec peak photon flux begins after the T90. 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/"