TITLE: GCN CIRCULAR NUMBER: 22493 SUBJECT: GRB 180313A: Fermi GBM detection DATE: 18/03/15 00:52:55 GMT FROM: Oliver J Roberts at USRA/NASA E. Bissaldi (Politecnico & INFN Bari), C. Meegan (UAH) and O.J. Roberts (USRA) report on behalf of the Fermi GBM Team: "At 23:28:17.53 UT on 13 March 2018, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 180313A (trigger 542676502 / 180313978). The on-ground calculated location, using the GBM trigger data, is RA, Dec = 317.51, -26.55 (J2000 degrees) with an uncertainty of 5.7 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 40 degrees. The GBM light curve shows/consists of single peak with a duration (T90) of about 0.1 s (50-300 keV). The time-averaged spectrum from T0+0 s to T0+0.13 s is best fit by a power-law function with an exponential high-energy cutoff. The power law index is -0.26 +/- 0.21 and the cutoff energy, parameterized as Epeak, is 287 +/- 44 keV The event fluence (10-1000 keV) in this time interval is (1.9 +/- 0.1)E-07 erg/cm^2. The 64 ms peak photon flux measured starting from T0+0 s in the 10-1000 keV band is 16 +/- 1 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."