TITLE: GCN CIRCULAR NUMBER: 20286 SUBJECT: GRB 161218B: Fermi GBM detection DATE: 16/12/18 18:19:00 GMT FROM: Rachel Hamburg at UAH R. Hamburg (UAH), C. Meegan (UAH), and H. Yu (MPE) report on behalf of the Fermi GBM Team: "At 08:32:40.65 UT on 18 December 2016, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 161218B (trigger 503742764 / 161218356). The on-ground calculated location, using the GBM trigger data, is RA = 358.64, DEC = -16.95, with an uncertainty of 1.00 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 80 degrees. The GBM light curve consists of multiple peaks with a duration (T90) of about 26 s (50-300 keV). The time-averaged spectrum from T0-2.0 s to T0+32.8 s is best fit by a Band function with Epeak = 214.80 +/- 2.51 keV, alpha = -0.51 +/- 0.01, and beta = -3.06 +/- 0.10. The event fluence (10-1000 keV) in this time interval is (8.91 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1.7 s in the 10-1000 keV band is 74.1 +/- 0.6 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."