TITLE: GCN CIRCULAR NUMBER: 23579 SUBJECT: GRB 181228A: Fermi GBM observation DATE: 18/12/28 14:35:02 GMT FROM: Elisabetta Bissaldi at INFN,Bari E. Bissaldi (Politecnico & INFN Bari) and P. Veres (UAH) report on behalf of the Fermi GBM Team: "At 02:53:14.96 UT on 28 December 2018, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 181228A (trigger 567658399 / 181228120), which was also detected by the Swift/BAT (Moss et al. 2018, GCN 23570). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time is 72 degrees. The GBM light curve consists of a two-peaked emission episode with a duration (T90) of about 18 s (50-300 keV). The time-averaged spectrum from T0 to T0+19 s is best fit by a Band function with Epeak = 182 +/- 8 keV, alpha = -0.82 +/- 0.03, and beta = -2.25 +/- 0.07. The event fluence (10-1000 keV) in this time interval is (2.56 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+15.6 s in the 10-1000 keV band is 16.0 +/- 0.4 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."