TITLE: GCN CIRCULAR NUMBER: 23183 SUBJECT: GRB 180828A: Fermi GBM observation DATE: 18/08/29 04:19:38 GMT FROM: Oliver J Roberts at USRA/NASA O.J. Roberts (USRA/NASA) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 18:57:26.58 UT on 28 August 2018, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 180828A (trigger 557175451 / 180828790), which was also detected by Swift (Beardmore et al. 2018, GCN 23182). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time using the Swift-XRT position is 77 degrees. The GBM light curve consists of multiple bright pulses with a duration (T90) of 8.3 s (50-300 keV). The time-averaged spectrum from T0+1.5s to T0+11 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.46 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 358 +/- 9 keV. A Band function fits the interval equally well, with Epeak =346 +/- 12 keV, alpha = -0.44 +/- 0.03 and beta = -3.01 +/- 0.33. The event fluence (10-1000 keV) over the T90 interval is (3.39 +/- 0.5)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0 +8s in the 10-1000 keV band is 38 +/- 1 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."