TITLE: GCN CIRCULAR NUMBER: 17511 SUBJECT: GRB 150301A: Fermi GBM observation DATE: 15/03/01 05:25:53 GMT FROM: Valerie Connaughton at UAH/NSSTC Valerie Connaughton and Peter Jenke (UAH) and Adam Goldstein (NASA MSFC) report on behalf of the Fermi GBM Team: "At 01:04:28.65 UT on 01 March 2015, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 150301A (trigger 446864671 / 150301045) which was also detected by the Swift/BAT (Lien et al. 2015, GCN 17510). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight is 105 degrees. The GBM light curve consists of a single peak about 50 ms in duration. The counts registered in the NaI detectors are strongly peaked in the 50 - 300 keV energy range typical for GRBs. From the quicklook data, the softness ratio for the event (20 - 100 keV / 100 - 500 keV) is 0.4 +/- 0.3, typical for fairly hard GRBs. By contrast, the corresponding softness ratios for recent triggers from SGR 1935+2154 and AXP 4U 0142+61 were 4.4 +/- 1.8, 3.4 +/- 1.0, and 2.2 +/- 0.4. The time-averaged spectrum from T0-0.016 s to T0+0.032 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.53 +/- 0.35 and the cutoff energy, parameterized as Epeak, is 185 +/- 49 keV. The event fluence (10-1000 keV) in this time interval is (1.2 +/- 0.2)E-07 erg/cm^2. The 16-msec peak photon flux measured starting from T0-0.016 s in the 10-1000 keV band is 29 +/- 4 ph/s/cm^2. The spectral characteristics of this burst are more like a GRB than an SGR. We think it is therefore unlikely that this event is a burst from a new SGR and conclude that it is, instead, a short GRB. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."