TITLE: GCN CIRCULAR NUMBER: 29668 SUBJECT: GRB 210318B: Fermi GBM detection DATE: 21/03/18 21:59:41 GMT FROM: Christian Malacaria at NASA-MSFC/USRA C. Malacaria (NASA-MSFC/USRA) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: At 03:08:46.50 UT on 18 March 2021, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 210318B (trigger 637729731 / 210318131), which was also detected by the Swift/BAT (Troja et al. 2019, GCN 29663) and by Konus-Wind (Svinkin et al. 2021, GCN 29664). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time is 91 degrees. The GBM light curve shows a peak with a duration (T90) of about 24.3 s (50-300 keV). The time-averaged spectrum from T0-4.1 s to T0+0.0 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is 2.72 +/- 1.53 and the cutoff energy, parameterized as Epeak, is 84.5 +/- 8.7 keV. The event fluence (10-1000 keV) in this time interval is (2.537 +/- 0.393)E-07 erg/cm^2. The 1-sec peak photon flux measured starting from T0-1.9 s in the 10-1000 keV band is 1.4 +/- 0.2 ph/s/cm^2. 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/