TITLE: GCN CIRCULAR NUMBER: 40107 SUBJECT: Konus-Wind detection of GRB 250406B (short/hard) DATE: 25/04/07 17:14:04 GMT FROM: Dmitry Frederiks at Ioffe Institute fred@mail.ioffe.ru

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

The bright, short-duration GRB 250406B (IPN localization: Kozyrev et al., GCN 40106). triggered Konus-Wind (KW) at T0=21401.479 s UT (05:56:41.479).

The burst light curve shows a multi-peaked pulse, which starts at ~T0-0.032 s and has a duration of ~0.240 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/GRB250406_T21401/

As observed by Konus-Wind, the burst had the total fluence of 2.54(-0.51,+0.62)x10^-6 erg/cm^2 and a 16-ms peak energy flux, measured from T0-0.016 s, of 3.06(-0.61,+0.69)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the burst (measured from T0 to T0+0.256 s) is best fit in the 20 keV - 15 MeV range by a power law with exponential cutoff (CPL) model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.97(-0.10,+0.12) and Ep = 2118(-590,+818) keV (chi2 = 35/38 dof). Fitting this spectrum by a Band function yields the same values of alpha and Ep, and an upper limit on the high energy photon index beta of -2.15 (chi2 = 35/37 dof).

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

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