TITLE:   GCN CIRCULAR
NUMBER:  29095
SUBJECT: Konus-Wind observation of GRB 201218A
DATE:    20/12/20 14:24:35 GMT
FROM:    Dmitry Svinkin at Ioffe Institute  <svinkin@mail.ioffe.ru>

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

The long-duration, very bright GRB 201218A
(Fermi-GBM detection: The Fermi GBM team, GCN Circ. 29081;
BALROG localization: Biltzinger et al., GCN Circ. 29083;
IPN triangulation: Hurley et al., GCN Circ. 29090)
triggered Konus-Wind at T0=15253.184 s UT (04:14:13.184).

The burst light curve shows a multipeaked structure
which starts at ~T0-17 s and has a total duration of ~55 s,
followed by a weak tail seen up to ~T0+120 s.
The emission is seen up to ~5 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB201218_T15255

As observed by Konus-Wind, the burst
had a fluence of 1.35(-0.07,+0.07)x10^-4 erg/cm2,
and a 64-ms peak flux, measured from T0+8.544 s,
of 2.23(-0.28,+0.29)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+20.736 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.82(-0.03,+0.03),
the high energy photon index beta = -3.00(-0.50,+0.27),
the peak energy Ep = 472(-24,+23) keV
(chi2 = 139/97 dof).

The spectrum near the maximum count rate
(measured from T0+8.192 to T0+8.704 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.72(-0.09,+0.09)
and Ep = 605(-68,+80) keV (chi2 = 51/63 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.5
(chi2 = 50/62 dof).

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