TITLE:   GCN CIRCULAR
NUMBER:  18788
SUBJECT: GRB 151231B: Fermi GBM observation
DATE:    15/12/31 19:42:06 GMT
FROM:    Peter Veres at UAH  <veresp@gmail.com>

P Veres (UAH) and C Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 13:38:08.17 UT on 31 December 2015, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 151231B (trigger 473261892 / 151231568).


The on-ground calculated location, using the GBM trigger
data, is RA = 150.1, DEC = 28.8 (J2000 degrees,
equivalent to 10 h 0 m, 28 d 49 '), with an uncertainty
of 3.1 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 angle from the Fermi LAT boresight at the GBM trigger time is 50 degrees.

The GBM light curve consists of a single pulse
with a duration (T90) of about 0.8 s (50-300 keV).
The time-averaged spectrum from T0-0.3 s to T0+0.4 s is
abest fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.65 +/- 0.10 and
the cutoff energy, parameterized as Epeak, is 419 +/- 58 keV.

The event fluence (10-1000 keV) in this time interval is
(1.20 +/- 0.07)E-6 erg/cm^2. The 64 ms peak photon flux measured
starting from T0 in the 10-1000 keV band is 14.8 +/- 1.2 ph/s/cm^2.

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