[vsnet-grb-info 20458] Konus-Wind observation of GRB 180318A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22554
SUBJECT: Konus-Wind observation of GRB 180318A
DATE:    18/03/27 16:06:20 GMT
FROM:    Anastasia Tsvetkova at Ioffe Institute  <tsvetkova at mail.ioffe.ru>

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

The long-duration GRB 180318A
(IPN Triangulation: Hurley et al., GCN 22522)
triggered Konus-Wind at T0=84002.795 s UT (23:20:02.795).

The burst light curve shows two emission episodes with multipeaked 
structure,
which start at ~T0-53 s and have a total duration of ~280.0 s.
The main episode lasts from ~T0-53 s to ~T0+30 s,
the second episode lasts from ~T0+150 s to ~T0+200 s.
The emission is seen up to 1.2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180318_T84002/

As observed by Konus-Wind, the burst
had a fluence of 1.13(-0.24,+0.23)x10^-4 erg/cm2,
and a 64-ms peak energy flux, measured from T0+18.560 s,
of 4.89(-1.04,+1.11)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst
(measured from T0 to T0+205.056 s)
is best fit in the 20 keV - 1.2 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.89(-0.24,+0.31),
the high energy photon index beta = -2.12(-0.48,+0.18),
the peak energy Ep = 195(-45,+69) keV,
chi2 = 83/59 dof.

The spectrum near the maximum count rate
(measured from T0+16.640 to T0+24.832 s)
is best fit in the 20 keV - 1.2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.98(-0.19,+0.22)
and Ep = 251(-40,+63) keV (chi2 = 63/60 dof).
Fitting by the GRB (Band) model yields the same alpha and Ep,
and  beta < -2.13 (chi2 = 62/59 dof)

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