TITLE: GCN CIRCULAR NUMBER: 21520 SUBJECT: GRB 170817A: Fermi GBM detection DATE: 17/08/17 20:00:07 GMT FROM: Andreas von Kienlin at MPE A. von Kienlin (MPE), C. Meegan (UAH) and A. Goldstein (USRA) report on behalf of the Fermi GBM Team: "At 12:41:06.47 UT on 17 August 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170817A (trigger 524666471 / 170817529). The on-ground calculated location, using the GBM trigger data, is RA = 176.8, DEC = -39.8 (J2000 degrees, equivalent to 12 h 47 m, -39 d 48'), with an uncertainty of 11.6 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 angle from the Fermi LAT boresight at the GBM trigger time is 91 degrees. The GRB light curve shows a weak short pulse with a duration (T90) of about 2 s (50-300 keV). The time-averaged spectrum from T0-0.512 s to 2.048 s is well fit by a power law function with an exponential high-energy cutoff. The power law index is -0.89 +/- 0.5 and the cutoff energy, parameterized as Epeak, is 82 +/- 21 keV The event fluence (10-1000 keV) in this time interval is (2.3 +/- 0.4)E-07 erg/cm^2. The 1.024-sec peak photon flux measured starting from T0-0.32 s in the 8-1000 keV band is 1.9 +/- 0.2 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."