GCN Circular 21573

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

"At 03:17:19.98 UT on 18 August 2017, the Fermi Gamma-Ray
Burst Monitor triggered and located GRB 170818A
(trigger 524719044 / 170818137). The on-ground calculated
location, using the GBM trigger data, is RA = 297.2,
DEC = +6.4, with an uncertainty of 11.5 degrees (radius,
1-sigma containment, statistical only; there is additionally
a systematic error which we have characterized as a
core-plus-tail model, with 90% of GRBs having a 3.7 deg error
and a small tail suffering a larger than 10 deg systematic error.
[Connaughton et al. 2015, ApJS, 216, 32] ).

The initial angle from the Fermi LAT boresight to
the GBM best location is 109 degrees.

The GBM triggered on a single pulse over a
total duration (T90) of about 0.6 s (50-300 keV).
The time-averaged spectrum from T0-0.06 to T0+0.51 s is
best fit by a power law function with an exponential
high-energy cutoff. The power law index is -0.37 +/- 0.32 and
the cutoff energy, parameterized as Epeak, is 80 +/- 9 keV

The event fluence (10-1000 keV) in this time interval is
(2.6 +/- 0.3)E-07 erg/cm^2. The 64 ms peak photon flux measured
starting from T0+0.0 s in the 10-1000 keV band
is 14 +/- 2 ph/s/cm^2.