TITLE: GCN CIRCULAR NUMBER: 10568 SUBJECT: GRB 100401A: Fermi GBM detection DATE: 10/04/02 22:59:08 GMT FROM: Adam Goldstein at Fermi-GBM/UAH A. Goldstein (UAH) and D. Gruber (MPE) report on behalf of the Fermi GBM Team: "At 07:07:32.24 UT on 1 April 2010, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 100401A (trigger 291798454 / 100401297) which was also detected by the Swift-BAT (J.R. Cummings et al. 2010, GCN 10567). The GBM on-ground location is consistent with the Swift-BAT position. The angle from the Fermi LAT boresight is 45 degrees. The GBM light curve consists of an initial bright pulse at trigger time lasting ~4 s followed by a second, much softer pulse of ~30 s in duration, approximately 65 s after trigger time. The total duration (T90) is estimated to be about 100 s (8-1000 keV). The time-averaged spectrum of the first pulse from T0-1.9 s to T0+3.2 s is best fit by a power law function with an exponential high energy cutoff. The power law index is -1.49 +/- 0.17 and the cutoff energy, parameterized as Epeak, is 144.90 +/- 62.20 keV (chi squared 439 for 483 d.o.f.). The first pulse is equally well fit by a simple power law with index -1.77 +/- 0.06 (chi squared 444 for 484 d.o.f.). The time-averaged spectrum of the second pulse from T0+74.9 s to T0+98.4 s is well fit by a power law function with an exponential high energy cutoff. The power law index is -1.52 +/- 0.39 and the cutoff energy, parameterized as Epeak, is 29.34 +/- 8.96 keV (chi squared 511 for 483 d.o.f.). The second pulse is equally well fit by a simple power law with index -2.16 +/- 0.11 (chi squared 514 for 484 d.o.f.). The event fluence (8-1000 keV) over the entire emission is (2.39 +/- 0.05)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0-1.4 s in the 8-1000 keV band is 3.61 +/- 0.16 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."