TITLE: GCN CIRCULAR NUMBER: 9850 SUBJECT: GRB 090831: Fermi GBM detection DATE: 09/08/31 10:11:30 GMT FROM: Arne Rau at MPE Arne Rau (MPE) reports on behalf of the Fermi GBM Team: "At 07:36:36.58 UT on 31 August 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090831 (trigger 273396998 / 090831317). The on-ground calculated location, using the GBM trigger data, is RA = 145.1, DEC = 51.0 (J2000 degrees, equivalent to 09 h 40m m, +51 d 00 '), with an uncertainty of 1.9 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 107 degrees. This burst was also independently detected by INTEGRAL SPI-ACS. The GBM light curve consists of two structured main pulses with a duration (T90) of about 69.1 s (8-1000 keV). The time-averaged spectrum from T+0.0 s to T0+47.7 s is best fit by a power law function with an exponential high energy cutoff. The power law index is -1.57 +/- 0.03 and the cutoff energy, parameterized as Epeak, is 399.6 +/- 86.2 keV (chi squared 443 for 415 d.o.f.). The event fluence (8-1000 keV) in this time interval is (1.66 +/- 0.05)E-05 erg/cm^2. The 1.024-sec peak photon flux measured starting from T0+8.784 s in the 8-1000 keV band is 9.4 +/- 0.4 ph/s/cm^2. A Band function fits the spectrum equally well (chi squared 441 for 414 d.o.f.) with Epeak= 243.8 +/- 85.3 keV, alpha = -1.52 +/- 0.06 and beta = -1.96 +/- 0.14. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."