TITLE: GCN CIRCULAR NUMBER: 29642 SUBJECT: Konus-Wind detection of GRB 210306A DATE: 21/03/10 10:05:25 GMT FROM: Anna Ridnaia at Ioffe Institute A. Ridnaia, S. Golenetskii, D. Frederiks, M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline on behalf of the Konus-Wind team, report: The long-duration GRB 210306A (Swift-BAT detection: D'Elia et al., GCN 29597, Laha et al., GCN 29617; Fermi-LAT detection: Ohno et al., GCN 29602; Fermi-GBM observation: Veres, GCN 29605) triggered Konus-Wind at T0=14040.731 s UT (03:54:00.731). The burst light curve shows a multi-peaked emission complex, which starts at ~T0-0.2 s and has a total duration of ~11 s. The emission is seen up to ~1 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB210306_T14040/ As observed by Konus-Wind, the burst had a fluence of 7.01(-0.45,+0.48)x10^-6 erg/cm2, and a 64-ms peak flux, measured from T0+7.648 s, of 3.99(-0.60,+0.67)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+16.640 s) is best fit in the 20 keV - 1 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.21(-0.20,+0.22) and Ep = 77(-7,+7) keV (chi2 = 48/49 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.6 (chi2 = 46/48 dof). The spectrum near the peak count rate (measured from T0+0.256 to T0+8.448 s) is best fit in the 20 keV - 1 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -1.17(-0.23,+0.30), the high energy photon index beta = -2.97(-1.31,+0.34), the peak energy Ep = 70(-7,+7) keV (chi2 = 56/49 dof). All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.