TITLE:   GCN CIRCULAR
NUMBER:  40485
SUBJECT: Konus-Wind detection of GRB 250516A
DATE:    25/05/18 13:53:49 GMT
FROM:    Dmitry Frederiks at Ioffe Institute <fred@mail.ioffe.ru>

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

The long GRB 250516A (Fermi GBM detection: Fermi GBM team, GCN 40466;
Mukherjee & Meegan, GCN 40473; Swift detection: Beardmore et al., GCN 40467)
was detected by Konus-Wind (KW) in the waiting mode.

A Bayesian block analysis of the KW waiting mode data in the 20-400 keV band
reveals a ~11 sigma count-rate increase in the interval
from T0-2.757 s to T0+23.739 s where T0 = 14:35:29 UT.

The KW light curve of this burst is available
at http://www.ioffe.ru/LEA/GRBs/GRB250516A/

Modeling the time-integrated spectrum of the burst
by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha = -1.21 (-0.10, + 0.10) and Ep = 164(-9,+10) keV.

Modelling the spectrum near the peak count rate (T0+3.131 s to T0+9.019 s)
by a CPL function yields alpha = -1.35 (-0.10, + 0.11) and Ep = 403(-53,+82) keV.

In the 20 keV -10 MeV band, standard for the KW analysis,
the burst fluence is (4.41 ± 1.33)x10^-6 erg/cm^2
and the 2.944 s peak energy flux, measured from T0+6.075 s,
is (5.85 ± 0.42)x10^-7 erg/cm^2/s.

All the quoted errors are estimated at the 68% confidence level.
All the presented results are preliminary.