TITLE:   GCN CIRCULAR
NUMBER:  40810
SUBJECT: Konus-Wind detection of GRB 250620C
DATE:    25/06/22 10:03:09 GMT
FROM:    Dmitry Svinkin at Ioffe Institute <dmitrysvinkin@gmail.com>

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

The bright,long-duration GRB 250620C
(Fermi-GBM detection: The Fermi GBM team, GCN 40794;
Mukherjee, GCN 40799;
SVOM-GRM detection: Wang et al., GCN 40807;
IPN triangulation: Svinkin et al., GCN 40809)
triggered Konus-Wind at T0=58026.172 s UT (16:07:06.172).

The burst light curve shows a multipeaked structure
which starts at ~T0-0.2 s and has a total duration of ~39 s.
The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250620_T58026/

As observed by Konus-Wind, the burst
had a fluence of 7.87(-0.47,+0.43)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+1.344 s,
of 4.05(-0.28,+0.28)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+39.424 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.00(-0.03,+0.03),
the high energy photon index beta = -2.83(-0.42,+0.21),
the peak energy Ep = 363(-17,+19) keV
(chi2 = 116/97 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+1.536 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.71(-0.05,+0.05),
the high energy photon index beta = -2.44(-0.14,+0.12),
the peak energy Ep = 618(-45,+47) keV
(chi2 = 85/59 dof).

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.