TITLE: GCN CIRCULAR NUMBER: 13757 SUBJECT: GRB 120911B: Fermi GBM observation DATE: 12/09/12 23:37:58 GMT FROM: David Gruber at MPE David Gruber (MPE), W. Paciesas (USRA), V. Pélassa (UAH) and V. Chaplin (UAH) report on behalf of the Fermi GBM Team: "We report on the ground analysis of the IPN detected GRB 120911B (GCN 13755) in the GBM data. The GBM light curve consists of a precursor starting at ~ 06:25:14 UT (T0) which is followed by a bright and hard main emission ~ 50 s later, lasting for another ~ 40 s. The duration (T90) is of about 69 s (50-300 keV). The time-averaged spectrum from T0 to T0+132 s is best fit by a Band function with Epeak = 1200 +/- 55 keV, alpha = -1.01 +/- 0.01, and beta = -2.72 +/- 0.14. The event fluence (10-1000 keV) in this time interval is (1.973 +/- 0.026)E-04 erg/cm^2. The 4-sec peak photon flux measured starting from T0+70 s in the 10-1000 keV band is 33.04 +/- 0.7 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."