TITLE: GCN CIRCULAR NUMBER: 40167 SUBJECT: Konus-Wind detection of GRB 250416C (a counterpart of EP250416a) DATE: 25/04/18 17:38:33 GMT FROM: Dmitry Svinkin at Ioffe Institute svinkin@mail.ioffe.ru

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

The long-duration GRB 250416C was detected by Konus-Wind (KW) in the waiting mode at about T0=64427 s UT (17:53:47).

A Bayesian block analysis of the KW waiting mode data in the 20-400 keV band reveals a ~7 sigma count-rate increase in the interval from T0-6.3 s to T0+11.4 s.

The KW light curve of this burst is available at http://www.ioffe.ru/LEA/GRBs/GRB250416C/

The total burst fluence is 1.50(-0.42,+1.04)x10^-6 erg/cm^2, and the 2.944 s peak energy flux, measured from T0-0.405 s, is 1.57(-0.57,+1.15)x10^-7 erg/cm^2. (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst, measured from T0-6.3 s to T0+11.4 s, can be described by a power law with exponential cutoff (CPL) model dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.35(-0.36,+0.66) and Ep = 228(-110,+735) keV.

The burst was detected simultaneously with the fast X-ray transient EP250416a (T0(EP)=17:53:59 UT; Zhao et al., GCN 40154; Zhou et al., GCN 40165). The positional and temporal coincidence of this burst with the transient supports the conclusion that the EP250416a is the GRB counterpart.

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

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