TITLE:   GCN CIRCULAR
NUMBER:  18200
SUBJECT: Konus-Wind observation of GRB 150821A
DATE:    15/08/24 15:32:49 GMT
FROM:    Anna Kozlova at Ioffe Institute  <ann_kozlova@mail.ioffe.ru>

S. Golenetskii, R. Aptekar, D. Frederiks, V. Pal'shin, 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-duration GRB 150821A (Swift-BAT trigger #652847:
Troja et al., GCN 18186; Stamatikos et al., GCN 18196;
Fermi GBM detection: Roberts, GCN 18190)
triggered Konus-Wind at T0=35074.166 s UT (09:44:34.166).

The burst light curve shows a single multi-peaked pulse
which started at ~T0-1.4 s and had a total duration of ~161.6 s.
The emission is seen up to ~10 MeV.

As observed by Konus-Wind, the burst
had a fluence of 9.16(-2.22,+2.38)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+4.768 s,
of 3.68(-1.02,+1.27)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+164.096 s)
is best fit in the 20 keV - 18 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.47 (-0.09,+0.15),
the high energy photon index beta = -2.21 (-7.79,+0.29),
the peak energy 350 (-125,+202) keV
(chi2 = 129/97 dof)

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 18 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.07 (-0.10,+0.15),
the high energy photon index beta = -3.13 (-6.87,+0.80),
the peak energy 327 (-66,+58) keV
(chi2 = 91/91 dof)

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB150821_T35074/
The background variations in the soft KW band are due to solar activity.

Assuming the redshift z=0.755 (D'Elia et al., GCN 18187)
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is ~1.4x10^53 erg,
the peak luminosity L_iso is ~1.0x10^52 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~614 keV.

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