//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 21777 SUBJECT: GRB 170831A: Fermi GBM detection DATE: 17/08/31 17:05:23 GMT FROM: Elisabetta Bissaldi at U.Innsbruk/IAPP E. Bissaldi (Politecnico & INFN Bari) reports on behalf of the Fermi GBM Team: "At 04:18:11.13 UT on 31 August 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170831A (trigger 525845896 / 170831179). The on-ground calculated location, using the GBM trigger data, is RA = 158.7, DEC = +49.8 (J2000 degrees, equivalent to 10h 34m, 49d 48'), with an uncertainty of 1.6 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 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 GBM ground location is 43 degrees. The GBM light curve consists of multiple peaks with a duration (T90) of about 60 s (50-300 keV). The time-averaged spectrum from T0-1.5 s to T0+55 s is adequately fit by a Band function with Epeak = 90 +/- 5 keV, alpha = -1.32 +/- 0.03, and beta = -2.52 +/- 0.12. The event fluence (10-1000 keV) in this time interval is (2.03 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+44 s in the 10-1000 keV band is 23.8 +/- 0.4 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."