TITLE: GCN CIRCULAR NUMBER: 15644 SUBJECT: GRB 131231A: Fermi GBM detection DATE: 14/01/01 00:42:48 GMT FROM: Peter Jenke at MSFC P. Jenke (UAH) and S. Xiong (UAH) reports on behalf of the Fermi GBM Team: "At 04:45:16.083 UT on December 31 2013, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 131231A (trigger 410157919/131231198). This GRB was detected in a ground analysis by the Fermi LAT (Sonbas et al. GCN 15640). It was also detected in optical follow-up observations of the GBM position by DARK/NBI (Xu et al. GCN 15641) and iPTF (Singer et al. GCN 15643), and confirmed by NOT observations of the Fermi LAT position (Malesani et al. GCN 15642). The on-ground calculated location from GBM is consistent with the positions reported from these follow-up observations. The trigger resulted in an Autonomous Repoint Request (ARR) that was accepted and the LAT slewed to the GBM in-flight location. The initial angle from the LAT boresight is 38 deg from Fermi/GBM position. The GBM light curve consists of a single large peak preceded by a smaller peak which resulted in the trigger. The duration (T90) of the burst was about 31 s (50-300 keV). The time-averaged spectrum from T0+.003 s to T0+56 s is best fit with a Band function with Epeak = 146 +/- 3 keV, alpha = -1.10 +/- 0.01 and beta = -2.14 +/- 0.01. The event fluence (10-1000 keV) in this time interval is (2.51 +/- 0.01)E-06 erg/cm^2. The 1.0-sec peak photon flux measured starting from T0+22 s in the 10-1000 keV band is 67.3 +/- 0.8 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."