TITLE: GCN CIRCULAR NUMBER: 21573 SUBJECT: GRB 170818A: Fermi GBM detection DATE: 17/08/18 19:25:23 GMT FROM: Oliver J Roberts at USRA/NASA O.J. Roberts (USRA) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 03:17:19.98 UT on 18 August 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170818A (trigger 524719044 / 170818137). The on-ground calculated location, using the GBM trigger data, is RA = 297.2, DEC = +6.4, with an uncertainty of 11.5 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a core-plus-tail model, with 90% of GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg systematic error. [Connaughton et al. 2015, ApJS, 216, 32] ). The initial angle from the Fermi LAT boresight to the GBM best location is 109 degrees. The GBM triggered on a single pulse over a total duration (T90) of about 0.6 s (50-300 keV). The time-averaged spectrum from T0-0.06 to T0+0.51 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.37 +/- 0.32 and the cutoff energy, parameterized as Epeak, is 80 +/- 9 keV The event fluence (10-1000 keV) in this time interval is (2.6 +/- 0.3)E-07 erg/cm^2. The 64 ms peak photon flux measured starting from T0+0.0 s in the 10-1000 keV band is 14 +/- 2 ph/s/cm^2.