TITLE:   GCN CIRCULAR
NUMBER:  26506
SUBJECT: Konus-Wind observation of GRB 191218A
DATE:    19/12/19 19:58:46 GMT
FROM:    Anna Kozlova at Ioffe Institute  <ann_kozlova@mail.ioffe.ru>

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

The long-duration GRB 191218A (Swift-BAT trigger #944916:
Troja et al., GCN 26480; Markwardt et al., GCN 26495;
AstroSat CZTI detection: Gaikwad et al., GCN 26482)
triggered Konus-Wind at T0=9758.735 s UT (02:42:38.735).

The burst light curve shows a double-peaked structure
which starts at ~T0-0.5 s and has a total duration of ~78.8 s.
The emission is seen up to ~4 MeV.

As observed by Konus-Wind, the burst
had a fluence of 3.92(-0.37,+0.45)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+1.440 s,
of 4.08(-1.07,+1.07)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+81.664 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.76(-0.18,+0.20)
and Ep = 330(-44,+61) keV (chi2 = 54/69 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.2
(chi2 = 53/68 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+7.936 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model
with  alpha = -0.27(-0.15,+0.17)
and Ep = 322(-24,+28) keV (chi2 = 74/69 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 73/68 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB191218_T09758/

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.