TITLE:   GCN CIRCULAR
NUMBER:  26100
SUBJECT: Konus-Wind observation of GRB 191027A
DATE:    19/10/30 20:03:53 GMT
FROM:    Anna Kozlova at Ioffe Institute  <ann_kozlova@mail.ioffe.ru>

A.Ridnaia, S. Golenetskii, R. Aptekar, D. Frederiks,
M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline
on behalf of the Konus-Wind team, report:

The long-duration GRB 191027A
(Fermi-GBM detection: The Fermi GBM team, GCN Circ. 26078;
IPN triangulation: Hurley et al., GCN Circ. 26087)
triggered Konus-Wind at T0=55143.905 s UT (15:19:03.905).

The burst light curve shows a single pulse
started at ~T0-0.1 s with a total duration of ~7.1 s.
The emission is seen up to ~4 MeV.

As observed by Konus-Wind, the burst
had a fluence of 5.24(-1.01,+1.84)x10^-6 erg/cm2,
and a 64-ms peak flux, measured from T0-0.002 s,
of 7.53(-2.28,+2.60)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 - 4 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.28(-0.22,+0.26)
and Ep = 443(-153,+468) keV (chi2 = 68/73 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.0
(chi2 = 68/72 dof).

The spectrum near the maximum count rate
(measured from T0 to T0+0.064 s)
is best fit in the 20 keV - 4 MeV range
by a power law with exponential cutoff model
with  alpha = -0.09(-0.61,+0.84)
and Ep = 444(-106,+210) keV (chi2 = 8/15 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.2
(chi2 = 8/14 dof).

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB191027_T55143/

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