TITLE: GCN CIRCULAR NUMBER: 22138 SUBJECT: GRB 171120A: Fermi GBM Detection DATE: 17/11/21 00:44:27 GMT FROM: Matthew Stanbro at UAH/Fermi M. Stanbro, C. Meegan, (UAH) and A von Kienlin (MPE) report on behalf of the Fermi GBM Team: "At 13:20:02.37 UT on 20 November 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 171120A (trigger 532876807 / 171120556) which was also detected by the Swift/BAT (A. Tohuvavohu et al. 2017, GCN 22133) and Fermi LAT (F. Longo et al. 2017, GCN 22136). The GBM on-ground location is consistent with the Swift 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 consists of 2 pulses with a duration (T90) of about 44 s (50-300 keV). The time-averaged spectrum from T0+0.002 s to T0+44.897 s is best fit by a Band function with Epeak = 141 +/- 12 keV, alpha = -0.91 +/- 0.06, and beta = -2.16 +/- 0.09. The event fluence (10-1000 keV) in this time interval is (1.80 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.10 s in the 10-1000 keV band is 35.69 +/- 0.51 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."