TITLE: GCN CIRCULAR NUMBER: 9446 SUBJECT: GRB 090528: Fermi GBM detection DATE: 09/05/30 15:34:09 GMT FROM: Andreas von Kienlin at MPE A. von Kienlin (MPE) reports on behalf of the Fermi GBM Team: "At 04:09:01.14 UT on 28 May 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090528 (trigger 265176543 / 090528173). The on-ground calculated location, using the GBM trigger data, is RA = 134.9, DEC = -35.8 (J2000 degrees, equivalent to 09h 00m, -35d 48'), with an uncertainty of 3.4 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which is currently estimated to be 2 to 3 degrees). The GBM on-ground location is within two sigma of the Vela X-1 position, nevertheless we think this event is due to a real gamma-ray burst because intensity, duration and spectrum are unusual for Vela X-1 observed outburst. The angle from the Fermi LAT boresight is 81 degrees. The GBM light consists of one major structured pulse with a duration (T90) of about 68 s (8-1000 keV). The time-averaged spectrum from T0-5.120 s to T0+29.696 s is well fit by a power law function with an exponential high energy cutoff. The power law index is -1.7 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 99 +/- 26 keV (chi squared 357 for 359 d.o.f.). The event fluence (8-1000 keV) in this time interval is (9.3 +/- 0.9)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+3.072 s in the 8-1000 keV band is 7.6 +/- 0.3 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."