TITLE: GCN CIRCULAR NUMBER: 9501 SUBJECT: GRB 090531B: Fermi GBM detection DATE: 09/06/10 01:07:56 GMT FROM: Sylvain Guiriec at UAH S. Guiriec (UAH) reports on behalf of the Fermi GBM Team: "At 18:35:56.49 UT on 31 May 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090531B (trigger 265487758 / 090531775) which was also detected by the SWIFT-BAT (Cummings et al. 2008, GCN 9461). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight is 25 degrees. The GBM light curve consists of a single peak with a duration (T90) of about 2 s (8-1000 keV). The time-averaged spectrum from T0-0.23 s to T0+0.77 s is adequately fit by a Band function with Epeak = 2166(+675-408) keV, alpha = -0.71(+0.10-0.10), and beta = -2.47(+0.25-0.46) (Castor C-Stat 780 for 602 d.o.f.). The event fluence (8-1000 keV) in this time interval is (6.20 +/- 0.18)E-07 erg/cm^2. The 1-sec peak photon flux measured starting from T0-0.23 s in the 8-1000 keV band is 1.49 +/- 0.04 ph/s/cm^2. The spectral and temporal analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."