TITLE: GCN CIRCULAR NUMBER: 22990 SUBJECT: GRB 180720C: Fermi GBM observation DATE: 18/07/21 05:39:40 GMT FROM: Oliver J Roberts at USRA/NASA O.J. Roberts (USRA/NASA) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 22:23:53.15 UT on 20 July 2018, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 180720C (trigger 553818238 / 180720933), which was also detected by Swift (S.J. LaPorte et al. 2018, GCN 22982). The GBM on-ground location is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time using the Swift-XRT position is 111 degrees. The GBM light curve consists of broad, single peak with a duration (T90) of about 23 s (50-300 keV). The time-averaged spectrum from T0-6.1 to T0+17.4 s is adequately fit by a power law function with an exponential high-energy cutoff. The power law index is -1.12 +/- 0.08 and the cutoff energy, parameterized as Epeak, is 583 +/- 161 keV. The event fluence (10-1000 keV) over this time interval is (2.83 +/- 0.05)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+4.3 s in the 10-1000 keV band is 2.8 +/- 0.3 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."