TITLE: GCN CIRCULAR NUMBER: 20827 SUBJECT: GRB 170306B: Fermi GBM detection DATE: 17/03/06 19:32:55 GMT FROM: Peter Veres at UAH P. Veres (UAH), C. Meegan (UAH) and A. von Kienlin (MPE) report on behalf of the Fermi GBM Team: "At 14:07:22.27 UT on 6 March 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170306B (trigger 510502047 / 170306588), which was also detected by Fermi/LAT (Omodei et al., GCN 20826). The GBM on-ground location is consistent with the LAT position. 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 GBM light curve shows multiple overlapping pulses with a duration (T90) of about 18.7 s (50-300 keV). The time-averaged spectrum from T0-3.8 s to T0+26.9 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.73 +/- 0.03 and the cutoff energy, parameterized as Epeak, is 289 +/- 10 keV. The event fluence (10-1000 keV) in this time interval is (3.07 +/- 0.06)E-5 erg/cm^2. The 1-sec peak photon flux measured starting from T0+8.7 s in the 10-1000 keV band is 12.4 +/- 0.3 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."