TITLE:   GCN CIRCULAR
NUMBER:  18433
SUBJECT: Konus-Wind observation of GRB 151021A
DATE:    15/10/21 17:41:03 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, intense GRB 151021A (Swift-BAT trigger #660671:
Melandri et al., GCN 18425; Krimm et al., GCN 18429)
triggered Konus-Wind at T0=5336.535 s UT (01:28:56.535).

The burst light curve shows a multi-peaked structure
started at ~T0-6.1 s with a total duration of ~100 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/GRB151021_T05336/

As observed by Konus-Wind, the burst
had a fluence of 7.66(-0.87,+0.97)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+22.256 s,
of 4.92(-1.44,+1.41)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+131.328 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 = -1.14 (-0.14,+0.15),
the high energy photon index beta = -2.46 (-0.32,+0.18),
the peak energy 170 (-18,+23) keV
(chi2 = 94/96 dof)

The spectrum near the maximum count rate
(measured from T0+16.640 to T0+33.024 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 = -0.89 (-0.19,+0.19),
the high energy photon index beta = -2.29 (-0.32,+0.14),
the peak energy 200 (-27,+46) keV
(chi2 = 73/96 dof)

Assuming the redshift z=2.330 (de Ugarte Postigo et al., GCN 18426)
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 ~1.0x10^54 erg,
the peak luminosity L_iso is ~2.2x10^53 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~666 keV.

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