TITLE: GCN CIRCULAR NUMBER: 9184 SUBJECT: GRB 090418C: Fermi GBM Detection DATE: 09/04/22 01:08:13 GMT FROM: Valerie Connaughton at MSFC Valerie Connaughton (UAH) reports on behalf of the Fermi GBM Team: "At 19:35:24.92 UT on 18 April 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090418C (trigger 261776126 / 090418816). The on-ground calculated location, using the GBM trigger data, is RA = 262.8, DEC = -28.3 (J2000 degrees, equivalent to 17 h 31 m, -28 d 18 '), with an uncertainty of 14.4 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 58 degrees. The GBM light curve shows a short, double-peaked burst with a duration (T90) of about 0.6 s (8-1000 keV). The time-averaged spectrum from T0-0.128 s to T0+0.256 s is well fit by a power law with index -1.27 +/- 0.04, though formally it is best fit by a power law function with an exponential high energy cutoff. The power law index in this case is -0.94 +/- 0.08 and the cutoff energy, parameterized as Epeak, lies above 1 MeV. The event fluence (8-1000 keV) in this time interval is (6.0 +/- 0.3)E-7 erg/cm^2. The 0.128-sec peak photon flux measured starting from T0-0.128 s in the 8-1000 keV band is 8.5 +/- 0.5 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."