TITLE: GCN CIRCULAR NUMBER: 39619 SUBJECT: Konus-Wind detection of GRB 250306A DATE: 25/03/07 22:30:19 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 long-duration GRB 250306A (Swift-BAT detection: Evans et al., GCN 39606; Astrosat-CZTI detection: Tembhurnikar et al., GCN 39611) triggered Konus-Wind at T0=59404.612 s UT (16:30:04.612).

The burst light curve shows a double-peaked structure which starts at ~T0-0.5 s and has a total duration of ~15 s. The emission is seen up to ~3 MeV.

The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250306_T59404/

As observed by Konus-Wind, the burst had a fluence of 2.32(-0.15,+0.16)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+1.120 s, of 1.40(-0.12,+0.12)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+18.176 s) is best fit in the 20 keV - 20 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.94(-0.08,+0.08), the high energy photon index beta = -2.78(-0.33,+0.19), the peak energy Ep = 179(-10,+11) keV (chi2 = 84/97 dof).

The spectrum near the maximum count rate (measured from T0+0.256 to T0+1.792 s) is best fit in the 20 keV - 20 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.62(-0.08,+0.09), the high energy photon index beta = -2.59(-0.13,+0.10), the peak energy Ep = 217(-13,+14) keV (chi2 = 55/54 dof).

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

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