TITLE: GCN CIRCULAR NUMBER: 22278 SUBJECT: GRB 171223A: Fermi GBM detection DATE: 17/12/24 05:11:48 GMT FROM: Matthew Stanbro at UAH/Fermi M. Stanbro and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 19:38:15.07 UT on 23 December 2017, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 171223A (trigger 535750700 / 171223818). The on-ground calculated location, using the GBM trigger data, is RA = 115.84, DEC = -33.48, with an uncertainty of 6.59 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 best location is 56 degrees. The GBM light curve consists of 1 pulse with a duration (T90) of about 0.38 s (50-300 keV). The time-averaged spectrum from T0-0.13 s to T0+0.32 s is best fit by a Band function with Epeak = 1156 +/- 193 keV, alpha = -0.67 +/- 0.07, and beta = -2.42 +/- 0.28. The event fluence (10-1000 keV) in this time interval is (1.98 +/- 0.06)E-06 erg/cm^2. The 64-millisec peak photon flux measured starting from T0+0.064 s in the 10-1000 keV band is 23.8 +/- 1.2 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."