GCN Circular 9295
Elisabetta Bissaldi (MPE) reports on behalf of the Fermi GBM Team: "At 13:15:11.06 UT on 28 April 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090428B (trigger 262617313 / 090428552). The on-ground calculated location, using the GBM trigger data, is RA = 0.8, DEC = +6.5 (J2000 degrees, equivalent to 00h 03m, 06d 30'), with an uncertainty of 4.4 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 101 degrees. The GBM light curve consists of a broad single peak with a duration (T90) of about 30 s (8-1000 keV). The time-averaged spectrum from T0-9.2 s to T0+15.3 s is adequately fit by a power law function with an exponential high energy cutoff. The power law index is -1.86 +/- 0.07 and the cutoff energy, parameterized as Epeak, is 65 +/- 16 keV (chi squared 279 for 241 d.o.f.). The event fluence (8-1000 keV) in this time interval is (5.2 +/- 0.3)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+3.1 s in the 8-1000 keV band is 10.1 +/- 0.7 ph/s/cm^2. A Band function fits the spectrum equally well (chi squared 278 for 240 d.o.f.) with Epeak = 53 +/- 19 keV, alpha = -1.81 +/- 0.13 and beta = -2.17 +/- 0.21. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."