TITLE: GCN CIRCULAR NUMBER: 8971 SUBJECT: GRB 090309B: Fermi GBM detection DATE: 09/03/10 16:12:00 GMT FROM: Alexander van der Horst at NASA/MSFC A.J. van der Horst (NASA/MSFC/ORAU) reports on behalf of the Fermi GBM Team: "At 18:25:07.19 UT on 9 March 2009, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 090309B (trigger 258315909 / 090309.767). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 168.4, Dec = -55.0 (J2000 degrees, equivalent to J2000 11h13m, -55d01'), with a statistical uncertainty of 2.6 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 26 degrees. The GBM light curve consists of two main peaks, with durations of ~10 and ~25 seconds, and no significant emission above background level in between for ~25 seconds. The time-averaged spectrum over the two peaks, from T0-4.1 to T0+5.1 s and from T0+32.8 to T0+54.3 s, is best fit by a power law with an exponential high-energy cutoff. The index is -1.52 +/- 0.10 and the cutoff energy, parameterized as Epeak, is 197 +/- 65 keV (chi squared 358 for 360 d.o.f.). The fluence (8-1000 keV) in this time interval is (4.7 +/- 0.4)E-6 erg/cm^2. The 1-sec peak photon flux measured starting from T0+37.9s in the 8-1000 keV band is 4.43 +/- 0.17 ph/s/cm^2. The spectral and temporal analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."