TITLE: GCN CIRCULAR NUMBER: 10115 SUBJECT: GRB 091031: Fermi GBM detection DATE: 09/11/01 17:36:50 GMT FROM: Sheila McBreen at MPE S. McBreen (UCD/MPE) and V. Chaplin (UAH) report on behalf of the Fermi GBM Team: "At 12:00:28.85 UT on 31 October 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 091031 (trigger 278683230 / 091031500). The on-ground calculated location, using the GBM trigger data, is RA = 70.6, DEC = -59.1 (J2000 degrees, equivalent to 04 h 42 m, -59 d 06 '), with an uncertainty of 1 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 22 degrees. The GBM light curve consists of a number of pulses with sub-structure and has a duration (T90) of about 35 s (8-1000 keV). The time-averaged spectrum from T0-0.003 s to T0+36.609 s is best fit by a Band function with Epeak = 503.10 (+47.30/-46.60) keV, alpha = -0.91 +/- 0.04, and beta = -2.34 (+0.19/-0.31) (CSTAT 674 for 602 d.o.f.). The event fluence (8-1000 keV) in this time interval is (2.05 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+6.9 s in the 8-1000 keV band is 7.5 +/- 0.3 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog." [GCN OPS NOTE(01nov09): Per author's request, the missing minus sign to the value for alpha was added.]