GCN Circular 9861

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:

GRB 090831A (Rau, GCN 9850
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 ; Matsuoka et al., GCN 9852
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 )
triggered Konus-Wind at T0=27393.820 s UT (07:36:33.820)

The burst started with a short (~250 ms) spike which resembles
a classical short/hard GRB. The spike is followed by a structured
profile with a total duration of ~50 s.
The Konus-Wind light curves of this GRB are available at
http://www.ioffe.ru/LEA/GRBs/GRB090831_T27393/

As observed by Konus-Wind, the burst had
a fluence of 1.33(-0.2,+0.25)x10-5 erg/cm2, and
a 16-ms peak flux, measured from T0,
of 9.3(-2.2,+3.2)x10-6 erg/cm2/s
(both in the 20 keV - 2 MeV energy range).

The time-integrated spectrum of the burst (from T0 to T0+49.408 s)
is well fitted (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 = -1.53(-0.14,+0.16),  and
Ep = 253(-69,+162)keV (chi2 = 69/61 dof).
Fitting by the GRB (Band) model yields to the same alpha and Ep
values with the high energy photon index beta of -9 (< -2.1)

The spectrum at the maximum count rate,
measured from T0 to T0 + 0.256s  (initial short pulse),
is well fitted (in the 20 keV - 2 MeV range)
by the GRB (Band) model with:
alpha = -1.28(-0.3,+0.7),
beta  = -2.5( < -1.7), and
Ep    = 273(-212,+331)keV (chi2 = 12/22 dof).

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