TITLE:   GCN CIRCULAR
NUMBER:  21398
SUBJECT: Konus-Wind observation of GRB 170728B
DATE:    17/07/31 15:51:16 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 GRB 170728B (Swift-BAT trigger 765130:
Cenko et al., GCN 21371; Ukwatta et al., GCN 21384;
Fermi-LAT detection: Yassine et al., GCN 21380;
Fermi-GBM observation: Stanbro et al., GCN 21383)
triggered Konus-Wind at T0=82998.412 s UT (23:03:18.412).

The burst light curve shows an initial short multi-peaked structure
with a duration of ~0.5 s, followed by a weaker emission
seen up to ~48 s. The emission is seen up to ~2 MeV.

As observed by Konus-Wind, the burst
had a fluence of 3.96(-0.75,+0.93)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0-0.016 s,
of 4.38(-0.91,+0.93)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+8.448 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.13(-0.37,+0.45)
and Ep = 160(-43,+103) keV (chi2 = 66/62 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.1
(chi2 = 66/61 dof)

The spectrum near the maximum count rate
(measured from T0+0.000 to T0+0.256 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model
with  alpha = -0.22(-0.33,+0.39)
and Ep = 166(-22,+26) keV (chi2 = 22/36 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.0
(chi2 = 22/35 dof)

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB170728_T82998/

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