TITLE:   GCN CIRCULAR
NUMBER:  40507
SUBJECT: Konus-Wind detection of GRB 250516B
DATE:    25/05/20 21:30:43 GMT
FROM:    Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>

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 250516B
(GECAM-B detection: Wang et al., GCN 40469;
NuSTAR-ACS detection: Grefenstette et al., GCN 40484;
Insight-HXMT-HE detection: Wang et al., GCN 40489;
GRBAlpha detection: Dafcikova et al., GCN 40496;
IPN triangulation: Svinkin et al., GCN 40506)
triggered Konus-Wind at T0=23492.766 s UT (06:31:32.766).

The burst light curve shows multi-peaked structure
which starts at ~T0-0.5 s and has a total duration of ~56 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/GRB250516_T23492/

As observed by Konus-Wind, the burst
had a fluence of 3.00(-0.07,+0.07)x10^-4 erg/cm2,
and a 64-ms peak flux, measured from T0+19.232 s,
of 2.16(-0.19,+0.19)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+60.416 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 = -0.80(-0.03,+0.03),
the high energy photon index beta = -2.52(-0.07,+0.06),
the peak energy Ep = 356(-10,+11) keV
(chi2 = 124/97 dof).

The spectrum near the maximum count rate
(measured from T0+18.688 to T0+19.712 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 = -0.82(-0.07,+0.07),
the high energy photon index beta = -2.92(-0.50,+0.26),
the peak energy Ep = 482(-37,+41) keV
(chi2 = 46/53 dof).

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