TITLE:   GCN CIRCULAR
NUMBER:  20304
SUBJECT: Konus-Wind observation of GRB 161218B
DATE:    16/12/20 10:06:29 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred@mail.ioffe.ru>

D. Frederiks, S. Golenetskii, R.Aptekar, P. Oleynik, M. Ulanov,
D. Svinkin, A. Tsvetkova, A.Lysenko, A. Kozlova, and T. Cline,
on behalf of the Konus-Wind team, report:

The long GRB 161218B (Fermi-GBM detection: Hamburg et al., GCN 20286;
POLAR observation: Wang et al., GCN 20288;
IPN triangulation: Svinkin et al., GCN 20303)
triggered Konus-Wind at T0=30761.134 s UT (08:32:41.134).

The light curve starts with a bright, short pulse followed by
a weaker emission; the total duration of the burst is ~30 s.
The emission is seen up to ~10 MeV.

As observed by Konus-Wind, the burst had a fluence of
(8.6 ± 0.4)x10^-5 erg/cm2 and a 64-ms peak energy flux,
measured from T0+1.344, of (2.7 ± 0.2)x10^-5 erg/cm2
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+34.560 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.42 (-0.08,+0.08),
the high energy photon index beta = -3.37 (-0.80,+0.31),
the peak energy Ep = 203 (-8,+8) keV,
chi2 = 106/97 dof.

The spectrum near the peak count rate (measured from T0+0.256
to T0+1.536 s) is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.08 (-0.18,+0.21),
the high energy photon index beta = -2.71 (-0.24,+0.18),
the peak energy Ep = 203 (-15,+16) keV,
chi2 = 69/68 dof.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB161218_T30761/

All the quoted errors are estimated at the 90% confidence level.
All the presented results are preliminary.