TITLE: GCN CIRCULAR NUMBER: 33509 SUBJECT: Konus-Wind detection of GRB 230304B DATE: 23/03/23 13:08:58 GMT FROM: Dmitry Svinkin at Ioffe Institute D. Svinkin, D. Frederiks, M. Ulanov, A. Tsvetkova, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, report: The long-duration GRB 230304B (Fermi-GBM detection: the Fermi GBM team, GCN Circ. 33396; GRBAlpha detection: Dafcikova et al., GCN Circ. 33398; AstroSat-CZTI detection: Navaneeth et al., GCN Circ. 33400; CALET-GBM detection: Kobayashi et al., GCN Circ. 33423; IPN triangulation: Kozyrev et al., GCN Circ. 33480) triggered Konus-Wind at T0=52553.382 s UT (14:35:53.382). The burst light curve shows a multi-peaked structure which starts at ~T0 and has a total duration of ~53 s. The emission is seen up to ~6 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB230304_T52553/ As observed by Konus-Wind, the burst had a fluence of 4.57(-0.41,+0.60)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+0.560 s, of 1.21(-0.18,+0.18)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+40.448 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -1.05(-0.09,+0.09), the high energy photon index beta = -3.34(-6.66,+0.68), the peak energy Ep = 299(-28,+32) keV (chi2 = 68/82 dof). The spectrum near the maximum count rate (measured from T0 to T0+7.680 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.89(-0.08,+0.09) and Ep = 352(-27,+31) keV (chi2 = 91/83 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.2 (chi2 = 91/82 dof). All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.