TITLE: GCN CIRCULAR NUMBER: 30099 SUBJECT: GRB 210528A: Fermi GBM detection DATE: 21/05/29 15:34:40 GMT FROM: Peter Veres at UAH P. Veres and C. Meegan (both UAH) report on behalf of the Fermi GBM Team: "At 14:03:46.16 UT on 28 May 2021, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 210528A (trigger 643903431 / 210528586). The on-ground calculated location, using the GBM trigger data, is reported in GCN #30090. The GBM light curve consists of a strong pulse with some overlapping structure and a duration (T90) of about 1 s (50-300 keV). The time-averaged spectrum from T0-0.3 s to T0+1.3 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.47 +/- 0.06 and the cutoff energy, parameterized as Epeak, is 210 +/- 9 keV. The event fluence (10-1000 keV) in this time interval is (3.90 +/- 0.12)E-6 erg/cm^2. The 64ms peak photon flux measured starting from T0 + 0.256 s in the 10-1000 keV band is 31.7 +/- 2.0 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/"