[vsnet-grb-info 19859] Konus-Wind observation of GRB 170921B

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21926
SUBJECT: Konus-Wind observation of GRB 170921B
DATE:    17/09/25 16:20:31 GMT
FROM:    Anna Kozlova at Ioffe Institute  <ann_kozlova at 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 170921B
(Fermi GBM detection: Bissaldi, GCN Circ. 21902;
IPN Triangulation: Hurley et al., GCN Circ. 21918)
triggered Konus-Wind at T0=14528.886 s UT (04:02:08.886).

The burst light curve shows a single pulse which
starts at ~T0-0.2 s and has a total duration of ~25 s.
This pulse passes into a weaker decaying tail,
which can be traced out to ~T0+164 s.
The emission is seen up to ~4 MeV.

As observed by Konus-Wind, the burst
had a fluence of 5.44(-0.22,+0.22)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+0.704 s,
of 6.75(-1.20,+1.21)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+22.528 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.42(-0.05,+0.05)
and Ep = 138(-6,+7) keV (chi2 = 76/72 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.3
(chi2 = 76/71 dof)

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+2.048 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model
with  alpha = -0.33(-0.13,+0.14)
and Ep = 318(-22,+25) keV (chi2 = 52/67 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.7
(chi2 = 52/66 dof)

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB170921_T14528/

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