TITLE: GCN CIRCULAR NUMBER: 10187 SUBJECT: GRB 091120: Fermi GBM observation DATE: 09/11/24 10:14:38 GMT FROM: David Gruber at MPE David Gruber (MPE) reports on behalf of the Fermi GBM Team: "At 04:34:40.23 UT on 20 November 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 091120 (trigger 280384482 / 091120191). The on-ground calculated location, using the GBM trigger data, is RA = 224.8, DEC = -24.8 (J2000 degrees, equivalent to 14h 59m, -24d 48'), with an uncertainty of 1.32 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which is currently estimated to be 2 to 3 degrees). The angle from the Fermi LAT boresight is 45 degrees. Moreover, this burst was bright enough to result in a Fermi spacecraft repointing maneuver. The GBM light curve consists of several peaks with a duration (T90) of about 52 s (8-1000 keV). The time-averaged spectrum from T0-3 s to T0+56 s is adequately fit by a power law function with an exponential high energy cutoff. The power law index is -1.17 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 129 +/- 3 keV. (C-stat 691 for 362 d.o.f.). The event fluence (8-1000 keV) in this time interval is (3.02 +/- 0.04)E-5 erg/cm^2. The 1-sec peak photon flux measured starting from T0+26.6 s in the 8-1000 keV band is 21.3 +/- 0.4 ph/s/cm^2. A Band function fits the spectrum equally well (C-stat 687 for 361 d.o.f.) with Epeak= 124 +/- 4 keV, alpha = -1.15 +/- 0.02 and beta = -2.98 +/- 0.27. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."