TITLE: GCN CIRCULAR NUMBER: 21717 SUBJECT: GRB 170826B: Fermi GBM detection DATE: 17/08/27 01:48:55 GMT FROM: Suraj Poolakkil at UAH S. Poolakkil (UAH) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 19:38:56.48 UT on 26 August 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 170826B (trigger 525469141/ 170826819). The on-ground calculated location, using the GBM trigger data, is RA = 327.7, DEC = -31.8, with an uncertainty of 1 degree (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 fluence 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 best location is 110 degrees. The GBM light curve shows multiple overlapping pulses with a duration (T90) of about 11 s (50-300 keV). The time-averaged spectrum from T0-2.0 to T0+20.5 s is best fit by a Band function with Epeak = 385 +/- 24 keV, alpha = -0.98 +/- 0.02, and beta = -2.24 +/- 0.10. The event fluence (10-1000 keV) in this time interval is (3.592 +/- 0.052)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+4.0 s in the 10-1000 keV band is 25.4 +/- 0.5 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog." -- Suraj Poolakkil Fermi GBM Graduate Research Assistant Dept. of Space Science University of Alabama in Huntsville