[vsnet-grb-info 10943] Konus-Wind observation of GRB 110918A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12362
SUBJECT: Konus-Wind observation of GRB 110918A
DATE:    11/09/20 11:18:01 GMT
FROM:    Dmitry Frederiks at Ioffe Institute  <fred at mail.ioffe.ru>

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:

The long exceptionally intense GRB 110918A
(localized by IPN: Hurley et al., GCN 12357)
triggered Konus-Wind at T0=77222.856s UT (21:27:02.856)

The burst is the most intense long GRB event in the history
of Konus-Wind observations since November, 1994.

The light curve of event started with an extremely bright hard short
pulse followed by three weaker partly overlapping peaks in the next 25 seconds.
The emission during this part of the burst is seen up to ~12 MeV.
A spectral lag between hard G3(1400-300 keV) and soft G1(25-90 keV)
instrument's light curves of the initial bright phase of the event
is estimated to a moderate value of ~0.090 s.

A weak decaying emission in the soft energy channel G1(25-90 keV)
continues till at least T0+250s when the measurements stopped.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB110918_T77222/

As observed by Konus-Wind the burst
had a fluence of 7.5(-0.2,+0.2)x10-4 erg/cm2,
and a 16-ms peak flux, measured from T0+0.368 s,
of 8.7(-0.4,+0.4)x10-4 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The best fit of the time-integrated spectrum of the burst
(measured from T0 to T0+69.376 s) is achieved
(in the 20 keV - 10 MeV range) with the GRB (Band) model, for which:
the low-energy photon index alpha = -1.2 (-0.1, +0.1),
the high energy photon index beta = -2.0 (-0.04, +0.03),
the peak energy Ep = 150(-17, +20) keV,
chi2 = 171/81 dof.

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+0.512 s)
is best fitted in the 20 keV - 10 MeV range
by the GRB (Band) model, for which:
the low-energy photon index alpha = -0.34 (-0.02, +0.02),
the high energy photon index beta = -2.5 (-0.1, +0.1),
the peak energy Ep = 1150(-110, +120) keV,
chi2 = 102/83 dof.

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