TITLE: GCN CIRCULAR NUMBER: 21383 SUBJECT: GRB 170728B: Fermi GBM detection DATE: 17/07/29 15:47:09 GMT FROM: Matthew Stanbro at UAH/Fermi M. Stanbro and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 23:03:19.43 UT on 28 July 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170728B (trigger 522975804 / 170728961) which was also detected by the Swift/BAT (S. B. Cenko et al. 2017, GCN 21371) and Fermi LAT (M. Yassine et al. 2017, GCN 21380). The GBM on-ground location is consistent with the Swift position. The Swift/BAT reported a short GRB in GCN 21371. Fermi GBM sees the initial short peak at trigger time but is followed up by several peaks up the ~50s later. The trigger resulted in an Autonomous Repoint Request (ARR) by the GBM Flight Software owing to the high peak flux of the GRB. This ARR was accepted and the spacecraft slewed to the GBM in-flight location. The initial angle from the Fermi LAT boresight to the best location is 30 degrees. The GBM light curve consists of several episodes with a duration (T90) of about 46 s (50-300 keV). The time-averaged spectrum from T0-.90 s to T0+47.23 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.10 +/- 0.10 and the cutoff energy, parameterized as Epeak, is 175 +/- 26 keV The event fluence (10-1000 keV) in this time interval is (4.59 +/- 0.34)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0-0.13 s in the 10-1000 keV band is 12.7 +/- 0.32 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 124 +/- 30 keV, alpha = -0.93 +/- 0.19 and beta = -2.12 +/- 0.22. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."