TITLE:   GCN CIRCULAR
NUMBER:  22981
SUBJECT: GRB 180720B: Fermi GBM observation
DATE:    18/07/20 22:50:18 GMT
FROM:    Oliver J Roberts at USRA/NASA  <oliver.roberts@nasa.gov>

O.J. Roberts (USRA/NASA) and C. Meegan (UAH) report on behalf 
of the Fermi GBM Team:

"At 14:21:39.65 UT on 20 July 2018, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 180720B (trigger 553789304 / 180720598),
which was also detected by Swift (Siegel et al. 2018, GCN 22973/22975) 
and the LAT (Bissaldi et al., GCN 22980). 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 50 degrees.

The GBM light curve consists of a very bright, FRED-like peak with 
numerous overlapping pulses with a duration (T90) of 49 s (50-300 keV).
The time-averaged spectrum from T0 to T0+55 s is best fit by a Band 
function with Epeak = 631 +/- 10 keV, alpha = -1.11 +/- 0.01 and 
beta = -2.30 +/- 0.03.

The event fluence (10-1000 keV) over the T90 interval is 
(2.985 +/- 0.001)E-04 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.4 s in the 10-1000 keV band is 125 +/- 1 ph/s/cm^2.


The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."