Konus-Wind detection of GRB 210305A

The long-duration GRB 210305A (Swift-BAT detection: D'Ai et al., GCN 29595, Krimm et al., GCN 29611; Fermi-GBM observation: Veres et al., GCN 29628) triggered Konus-Wind at T0=68578.330 s UT (19:02:58.330). The burst light curve shows a multipeaked structure which starts at ~T0-20 s and has the total duration of ~78 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/GRB210305_T68578/ As observed by Konus-Wind, the burst had a fluence of 1.63(-0.31,+0.43)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+4.224 s, of 1.23(-0.51,+0.51)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+57.600 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 = -0.33(-0.65,+1.71), the high energy photon index beta = -2.45(-0.73,+0.30), the peak energy Ep = 84(-26,+25) keV (chi2 = 75/49 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 a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.54(-0.32,+0.37) and Ep = 133(-16,+21) keV (chi2 = 44/50 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.3 (chi2 = 42/49 dof). All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.