TITLE: GCN CIRCULAR NUMBER: 9698 SUBJECT: GRB 090720B: Fermi GBM detection DATE: 09/07/22 13:57:40 GMT FROM: Michael Burgess at UAH J. M. Burgess (UAH), A. Goldstein (UAH) and A. J. van der Horst (NASA/MSFC/ORAU) report on behalf of the Fermi GBM Team: "At 17:02:56.91 UT on 20 July 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090720B (trigger 269802178 / 090720710). The on-ground calculated location, using the GBM trigger data, is RA = 203.0, DEC = -54.8 (J2000 degrees, equivalent to 13h 32m, -54d 48'), with an uncertainty of 2.9 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 56 degrees. This burst was also independently detected by INTEGRAL SPI-ACS. The GBM light curve consists of two bright peaks with a duration (T90) of about 20 s (8-1000 keV). The time-averaged spectrum from T0+0.0 s to T0+20.1 s is well fit by a power law function with an exponential high energy cutoff. The power law index is -1.01 +/- 0.05 and the cutoff energy, parameterized as Epeak, is 982 +/- 186 keV (chi squared 484 for 483 d.o.f.). The event fluence (8-1000 keV) in this time interval is (1.06 +/- 0.03)E-5 erg/cm^2. The 1-sec peak photon flux measured starting from T0+5.8 s in the 8-1000 keV band is 10.9 +/- 0.4 ph/s/cm^2. A Band function fits the spectrum equally well (chi squared 482 for 482 d.o.f.) with Epeak= 924 +/- 201 keV, alpha = -1.00 +/- 0.05 and beta = -2.43 +/- 0.47. The temporal and spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."