TITLE: GCN CIRCULAR NUMBER: 21857 SUBJECT: Konus-Wind observation of GRB 170906C DATE: 17/09/11 15:53:01 GMT FROM: Anastasia Tsvetkova at Ioffe Institute A. Tsvetkova, S. Golenetskii, R.Aptekar, D. Frederiks, P. Oleynik, M. Ulanov, D. Svinkin, A. Lysenko, A. Kozlova and T. Cline on behalf of the Konus-Wind team, report: The long-duration GRB 170906C (Swift-BAT trigger #770981: Siegel et al., GCN 21826; Lien et al., GCN 21832) triggered Konus-Wind at T0=26780.985 s UT (07:26:20.985). The burst light curve shows a single pulse started at ~T0-40.8 s with a total duration of ~65.8 s. The emission is seen up to ~4 MeV. As observed by Konus-Wind, the burst had a fluence of  1.60(-0.19,+0.28)x10^-5 erg/cm2, and a 64-ms peak energy flux, measured from T0+0.354 s, of 2.77(-0.73,+0.74)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-integrated spectrum of the burst (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 4 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with  alpha = -1.06(-0.18,+0.19), and Ep = 154(-19,+26) keV (chi2 = 64/72 dof). Fitting by the GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.56 (chi2 = 64/71 dof). The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB170906_T26780/ All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.