TITLE: GCN CIRCULAR NUMBER: 9229 SUBJECT: GRB 090423: Fermi GBM observation DATE: 09/04/24 17:04:54 GMT FROM: Andreas von Kienlin at MPE A. von Kienlin (MPE) reports on behalf of the Fermi GBM Team: "At 07:55:25.39 UT on 23 April 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090423 (trigger 262166127 / 090423330) which was also detected by the Swift (H. A. Krimm et al. 2008, GCN 9198) The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight is 75.6 degrees. The GBM light curve shows a single structured peak with a duration (T90) of about 12 s (8-1000 keV). There is an indication for extended emission until 30 s after the burst onset. The time-averaged spectrum from T0-7.040 s to T0+5.248 s is well fit by a power law function with an exponential high energy cutoff. The power law index is -0.77 +/- 0.35 and the cutoff energy, parameterized as Epeak, is 82 +/- 15 keV (chi squared 275 for 238 d.o.f.). At a redshift of about 8 (F. Olivares et al. 2008, GCN 9215; C. Thoene et al. 2008, GCN 9216; N. Tanvir et al. 2008, GCN 9219), the Epeak in the GRB rest frame, Epeak_rest, is 738 +/- 135 keV. The event fluence (8-1000 keV) in this time interval is (1.1 +/- 0.3)E-06 erg/cm^2. Using standard cosmology (Omega_matter = 0.27, Omega_lambda = 0.73, H0=71) the isotropic equivalent energy in the 8-1000 keV band is E_iso = (8.9 +/- 2.4)E+53 ergs. The 1-sec peak photon flux measured starting from T0-1.920 s in the 8-1000 keV band is 3.3 +/- 0.5 ph/s/cm^2. The spectrum can also be fit by using a Band function with Epeak = 54 +/- 22 keV and beta = -2.1 +/- 0.3. However alpha is poorly constrained. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."