TITLE: GCN CIRCULAR NUMBER: 9294 SUBJECT: GRB 090428: Fermi GBM detection DATE: 09/04/29 15:20:35 GMT FROM: Elisabetta Bissaldi at MPE Elisabetta Bissaldi (MPE) reports on behalf of the Fermi GBM Team: "At 10:34:38.46 UT on 28 April 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090428 (trigger 262607680 / 090428441). The on-ground calculated location, using the GBM trigger data, is RA = 212.4, DEC = +37.3 (J2000 degrees, equivalent to 14h 10m, 37d 18'), with an uncertainty of 3.5 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 96 degrees. The GBM light curve consists of two peaks with a duration (T90) of about 8 s (8-1000 keV). The time-averaged spectrum from T0-0.51 s to T0+2.04 s is adequately fit by a power law function with an exponential high energy cutoff. The power law index is -0.59 +/- 0.16 and the cutoff energy, parameterized as Epeak, is 97 +/- 7 keV (chi squared 471 for 485 d.o.f.). The event fluence (8-1000 keV) in this time interval is (9.9 +/- 1.4)E-07 erg/cm^2. The 128-msec peak photon flux measured starting from T0+1.4 s in the 8-1000 keV band is 12.3 +/- 1.9 ph/s/cm^2. A Band function fits the spectrum equally well (chi squared 468 for 484 d.o.f.) with Epeak = 85 +/- 10 keV, alpha= -0.40 +/- 0.25 and beta = -2.7 +/- 0.4. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."