TITLE: GCN CIRCULAR NUMBER: 23128 SUBJECT: Konus-Wind observation of GRB 180809B DATE: 18/08/11 16:06:36 GMT FROM: Dmitry Svinkin at Ioffe Institute D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, M. Ulanov, A. Tsvetkova, A. Lysenko, A.Kozlova, and T. Cline on behalf of the Konus-Wind team, report: The long-duration GRB 180809B (Swift-BAT detection: Moss et al., GCN Circ. 23105; CALET-GBM detection: Cherry, et al., GCN Circ. 23116) triggered Konus-Wind at T0=73730.033 s UT (20:28:50.033). The burst light curve shows a bright multi-peaked emission episode which starts at ~T0-4 s and has a duration of ~80 s, followed by a weaker episode at ~T0-205 s. The total burst duration is ~300 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/GRB180809_T73730/ As observed by Konus-Wind, the burst had a fluence of 3.07(-0.30,+0.36)x10^-4 erg/cm2, and a 64-ms peak flux, measured from T0+37.952 s, of 2.66(-0.41,+0.42)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+252.672 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 = -0.87(-0.10,+0.12), the high energy photon index beta = -2.44(-0.24,+0.19), the peak energy Ep = 275(-30,+25) keV (chi2 = 123/97 dof). The spectrum near the maximum count rate (measured from T0+36.608 to T0+38.144 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 = -0.77(-0.12,+0.15), the high energy photon index beta = -2.20(-0.21,+0.15), the peak energy Ep = 525(-95,+103) keV (chi2 = 66/68 dof). All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.