TITLE:   GCN CIRCULAR
NUMBER:  12301
SUBJECT: Konus-Wind observation of GRB 110820B
DATE:    11/08/23 09:34:20 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred@mail.ioffe.ru>

S. Golenetskii, R.Aptekar, D. Frederiks, E. Mazets, V. Pal'shin,
P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf
of the Konus-Wind team, report:

The long hard extremely intense GRB 110820B
(localized by IPN: Hurley et al., GCN 12299)
triggered Konus-Wind at T0=77268.054s UT (21:27:48.054)

The event light curve shows two major bursting episodes
separated by ~100 s. A weaker and softer emission is
observed at the time between the two main pulses.
The total duration of the burst is ~200s.
The emission is seen up to ~10 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB110820_T77268/

As observed by Konus-Wind the burst
had a fluence of (2.5 ± 0.3)x10-4 erg/cm2,
and a 64-ms peak flux, measured from T0+3.136 s,
of (2.6 ± 0.4)x10-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+191.488 s) is best fitted
in the 20 keV - 10 MeV range by the GRB (Band)
model, for which:
the low-energy photon index alpha = -1.22 (-0.05, +0.06),
the high energy photon index beta = -2.1 (-0.3, +0.2),
the peak energy Ep = 481(-83, +90) keV,
chi2 = 69/84 dof.

The spectrum at the maximum count rate
(measured from T0+0.256 to T0+3.840 s) is best fitted
in the 20 keV - 10 MeV range by the GRB (Band)
model, for which:
the low-energy photon index alpha = -0.55 (-0.09, +0.09),
the high energy photon index beta = -2.5 (-0.4, +0.2),
the peak energy Ep = 483(-53, +60) keV,
chi2 = 81/84 dof.

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