TITLE: GCN CIRCULAR NUMBER: 30913 SUBJECT: GRB 210928A: Fermi GBM observation DATE: 21/10/01 15:23:19 GMT FROM: Cori Fletcher at USRA C. Fletcher (USRA) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 02:00:31.01 UT on 28 September 2021, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 210928A (trigger 654487236 / 210928084) which was also detected by the Fermi LAT (M. Axelsson et al. 2021, GCN 30912). The GBM on-ground location is consistent with the LAT position. The angle from the Fermi LAT boresight at the GBM trigger time is 16 degrees. The GBM light curve consists of a multiple peaks with a duration (T90) of about 24 s (50-300 keV). The time-averaged spectrum from T0+2.1 s to T0+26.6 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.45 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 465 +/- 10 keV. The event fluence (10-1000 keV) in this time interval is (4.925 +/- 0.052)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+18.04 s in the 10-1000 keV band is 10.6 +/- 0.3 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/" FSSC: Data » Data Access » GBM GBM Data Products. This page lists the science data products created by the GIOC (GBM Instrument Operations Center) and provided to the FSSC. GBM Daily, Trigger, and Burst Data fermi.gsfc.nasa.gov