TITLE: GCN CIRCULAR NUMBER: 18137 SUBJECT: Konus-Wind observation of GRB 150811B DATE: 15/08/13 13:43:58 GMT FROM: Anastasia Tsvetkova at Ioffe Institute S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik, M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, and T. Cline on behalf of the Konus-Wind team, report: The short-duration, hard-spectrum, intense GRB 150811B (Fermi GBM detection: Jenke, GCN 18130; IPN triangulation: Golenetskii et al., GCN 18133) triggered Konus-Wind at T0=73329.515 s UT (20:22:09.515). The burst light curve shows a single pulse with a duration of ~0.7 s. The emission is seen up to ~10 MeV. As observed by Konus-Wind, the burst had a fluence of 9.08(-1.11,+1.22)x10^-6 erg/cm2, and a 16-ms peak energy flux, measured from T0+0.232 s, of 2.89(-0.92,+0.95)x10^-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-integrated spectrum of the burst (measured from T0 to T0+0.256 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 = -0.35(-0.24,+0.29), and Ep = 1197(-220,+299) keV (chi2 = 28/27 dof). Fitting by the GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.51 (chi2 = 28/26 dof). The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB150811_T73329/ All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.