TITLE: GCN CIRCULAR NUMBER: 30913 SUBJECT: GRB 210928A: Fermi GBM observation DATE: 21/10/01 15:23:19 GMT FROM: Cori Fletcher at USRA cfletcher@usra.edu

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 � GBMhttps://fermi.gsfc.nasa.gov/ssc/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