GCN Circular 21404

M. Stanbro and C. Meegan (UAH) report on behalf of the Fermi GBM Team:

"At 15:18:24.80 UT on 02 August 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170802A (trigger 523379909 / 170802638).
The on-ground calculated location, using the GBM trigger
data, is RA = 52.3, DEC = -39.2, with an uncertainty
of 2.1 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 trigger resulted in an Autonomous Repoint Request (ARR)
by the GBM Flight Software owing to the high peak flux
of the GRB. This ARR was accepted and the spacecraft slewed to the GBM
in-flight
location. The initial angle from the Fermi LAT boresight to
the GBM best location is 41 degrees.

The GBM triggered on a precursor that
was followed ~1 s later by a brighter peak. The
total duration (T90) was about 2 s (50-300 keV).
The time-averaged spectrum from T0-0.13 s to T0+2.24 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.80 +/- 0.08 and
the cutoff energy, parameterized as Epeak, is 952 +/- 200 keV.

The event fluence (10-1000 keV) in this time interval is
(2.70 +/- 0.11)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.41 s in the 10-1000 keV band
is 8.20 +/- 0.27 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 670 +/- 163 keV, alpha = -0.70 +/- 0.11 and beta = -2.09 +/-
0.25.


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