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GCN Circular 22822

Subject
Konus-Wind observation of GRB 180618A (a short GRB with extended emission)
Date
2018-06-22T10:32:51Z (6 years ago)
From
Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>
D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks,
M. Ulanov, A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline
on behalf of the Konus-Wind team, report:

The bright, short-duration, hard GRB 180618A
(Swift-BAT detection: LaPorte et al., GCN Circ 22790;
Fermi-GBM detection: Hamburg et al., GCN Circ 22794)
triggered Konus-Wind at T0=2591.201 s UT (00:43:11.201).

The burst light curve shows a bright multi-peaked initial pulse
with a total duration of ~0.250 s, followed by
an extended emission seen up to ~T0+10 s.
The emission of the initial pulse is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180618_T02591/

As observed by Konus-Wind, the burst
had a fluence of 6.24(-1.13,+1.20)x10^-6 erg/cm2,
and a 16-ms peak flux, measured from T0-0.010 s,
of 5.60(-1.83,+1.88)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the initial pulse
(measured from T0 to T0+0.192 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.36(-0.24,+0.30)
and Ep = 2461(-565,+788) keV (chi2 = 47/38 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.4
(chi2 = 47/37 dof).

The time-averaged spectrum of the extended emission
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 2 MeV range
by the CPL model with alpha = -0.68(-0.79,+1.59),
Ep = 170(-66,+203) keV (chi2 = 102/98 dof).

Among ~300 bursts from the Konus catalog of short GRBs
(Svinkin et al., 2016, ApJS 224, 10), GRB 180618A is within top 1%
in the terms of 16-ms peak energy flux and within top 15% in the terms
of the total energy fluence, both in the 20 keV - 10 MeV range.
In the terms of Epeak, the burst is within 1% hardest
GRBs in the sample.

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