GCN Circular 22278

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

"At 19:38:15.07 UT on 23 December 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 171223A (trigger 535750700 / 171223818).

The on-ground calculated location, using the GBM trigger
data, is RA = 115.84, DEC = -33.48, with an uncertainty
of 6.59 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 best location is 56 degrees.

The GBM light curve consists of 1 pulse
with a duration (T90) of about 0.38 s (50-300 keV).
The time-averaged spectrum from T0-0.13 s to T0+0.32 s is
best fit by a Band function with Epeak = 1156 +/- 193 keV,
alpha = -0.67 +/- 0.07, and beta = -2.42 +/- 0.28.

The event fluence (10-1000 keV) in this time interval is
(1.98 +/- 0.06)E-06 erg/cm^2. The 64-millisec peak photon flux measured
starting from T0+0.064 s in the 10-1000 keV band
is 23.8 +/- 1.2 ph/s/cm^2.

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