TITLE: GCN CIRCULAR NUMBER: 10226 SUBJECT: GRB 091123: Fermi GBM detection of a very long burst DATE: 09/12/01 18:16:00 GMT FROM: Sheila McBreen at MPE S. McBreen (UCD/MPE), V. Connaughton (UAH), C. Wilson-Hodge (NASA MSFC), M. Finger (USRA) and D. Tierney (UCD) report on behalf of the Fermi GBM Team: "At 07:08:37.26 UT on 23 November 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 091123 (trigger 280652919 / 091123298). The on-ground calculated location, using the GBM trigger data, is RA = 297.1, DEC = -29.2 (J2000 degrees, equivalent to 19 h 48 m, -29 d 12'), with an uncertainty of 2.4 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 106 degrees. This burst was also independently detected by INTEGRAL SPI-ACS and Konus-Wind. The GBM light curve consists of 4 main emission phases, each containing structure, separated by long periods of quiescence. Comparison with Konus-Wind data shows that the the event was already on-going when when it rose above the Earth's horizon. This allows a further constraint to be placed on the possible arrival direction of this burst. A plot showing the 3-sigma localization contours (statistical only) and the limb of the Earth at T0, T0-5, and T0+5 s is available at: http://gammaray.nsstc.nasa.gov/gbm/science/loc_091123298.gif In the GBM data alone, the burst has a duration of at least 650 sec (8-1000 keV). The time-averaged spectrum from T0-5 s to T0+650 s is adequately fit by a power law function with an exponential high energy cutoff. The power law index is -1.08 +/- 0.05 and the cutoff energy, parameterized as Epeak, is 101.30 (+5.16/-4.52) keV (C-stat 1008 for 355 d.o.f.). There is significant spectral evolution within the fitted time interval. The event fluence (8-1000 keV) in this time interval is (4.07 +/- 0.09)E-05erg/cm^2. The 1-sec peak photon flux measured starting from T0+11.2 s in the 8-1000 keV band is 6.1 +/- 0.2 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog." We thank Valentin Pal'shin for providing the Konus-Wind lightcurves and informing us of the burst occultation by the Earth before the GBM trigger time.