TITLE: GCN CIRCULAR NUMBER: 9829 SUBJECT: GRB 090820: Fermi GBM Detection of a bright burst. DATE: 09/08/20 06:58:15 GMT FROM: Valerie Connaughton at MSFC Valerie Connaughton (UAH) reports on behalf of the Fermi GBM Team: "At 00:38:16.19 UT on 20 August 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090820027 (trigger 272421498 / 090820027). The on-ground calculated location, using the GBM trigger data, is RA = 87.7, DEC = 27.0 (J2000 degrees, equivalent to 05h 51 m, 27 d 00 '), with an uncertainty of 1.0 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 108 degrees. Although this burst was bright enough to result in a Fermi spacecraft repointing maneuver and ensuing pointed observations of the burst position, Earth avoidance constraints prevented such a maneuver until 3100 sec after the burst trigger. The light curve shows GBM triggered on a weak precursor, followed by a bright pulse beginning 30 s later and lasting a further 30 s. The time-averaged spectrum from T0+28 s to T0+60 s is best fit by a Band function with Epeak = 215 +/- 3 keV, alpha = -0.69 +/- 0.01, and beta = -2.61 - 0.05. The event fluence (8-1000 keV) in this time interval is (6.6 +/- 0.1)E-5 erg/cm^2. The 1-sec peak photon flux measured starting from T0+34 s in the 8-1000 keV band is about 58 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."