TITLE:   GCN CIRCULAR
NUMBER:  22825
SUBJECT: Konus-Wind observation of GRB 180620B
DATE:    18/06/22 15:28:00 GMT
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 long-duration GRB 180620B
(Swift-BAT detection: Evans et al., GCN Circ. 22807;
Fermi-GBM detection: Mailyan, GCN Circ. 22813)
triggered Konus-Wind at T0=57039.332 s UT (15:50:39.332).

The burst light curve shows a multi-peaked structure
which starts at ~T0-156 s and has a total duration of ~260 s.
The emission is seen up to 0.8 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB180620_T57039/

As observed by Konus-Wind, the burst
had a fluence of 1.19(-0.12,+0.14)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0-0.056 s,
of 1.52(-0.61,+0.72)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The spectrum integrated over the most intense
part of the burst (measured from T0 to T0+49.408 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.04(-0.31,+0.36)
and Ep = 118(-16,+23) keV (chi2 = 44/57 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 44/56 dof).

The spectrum near the maximum count rate
(measured from T0 to T0+8.448 s)
is best fit in the 20 keV - 2 MeV range
by a power law with exponential cutoff model:
with  alpha = -0.74(-0.33,+0.37)
and Ep = 129(-15,+21) keV (chi2 = 66/57 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.7
(chi2 = 66/56 dof).

Assuming the redshift z=1.1175 (Izzo et al., GCN Circ. 22823)
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.27, and Omega_Lambda = 0.73,
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is ~3.9x10^52 erg,
the peak luminosity L_iso is ~1.0x10^52 erg/s,
and the rest-frame peak energy of the time-integrated spectrum,
Ep,i, is ~273 keV.

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