TITLE:   GCN CIRCULAR
NUMBER:  19842
SUBJECT: Konus-Wind observation of GRB 160821A
DATE:    16/08/22 16:40:49 GMT
FROM:    Anna Kozlova at Ioffe Institute  <ann_kozlova@mail.ioffe.ru>

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

The long-duration, very bright GRB 160821A
(Swift detection: Siegel et al., GCN 19830; Fermi LAT detection:
McEnery et al., GCN 19831, Arimoto et al., GCN 19836;
Fermi GBM observation: Stanbro et al., GCN 19835)
triggered Konus-Wind at T0=74179.687 s UT (20:36:19.687).

The burst light curve shows a weak emission starting at ~T0-113 s
followed by the bright main episode at ~T0. A total duration of
the burst is ~320 s. 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/GRB160821_T74179/

As observed by Konus-Wind, the burst
had a fluence of 1.17(-0.03,+0.03)x10^-3 erg/cm2,
and a 64-ms peak flux, measured from T0+22.320 s,
of 1.06(-0.13,+0.13)x10^-4 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+99.328 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.00(-0.02,+0.02),
the high energy photon index beta = -1.99(-0.04,+0.04),
the peak energy Ep = 710(-44,+47) keV
(chi2 = 212/88 dof)

The spectrum near the maximum count rate
(measured from T0+22.272 to T0+22.528 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 = -0.84(-0.10,+0.12),
the high energy photon index beta = -1.89(-0.29,+0.17),
the peak energy Ep = 1074(-326,+431) keV
(chi2 = 54/57 dof)

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