TITLE: GCN CIRCULAR NUMBER: 20005 SUBJECT: Konus-Wind observation of GRB 161004B DATE: 16/10/06 16:02:46 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 161004B (Swift-BAT trigger #715246: D'Ai et al., GCN 19987; Sakamoto et al., GCN 19995; Fermi GBM detection: Hamburg & Meegan, GCN 19998; CALET Gamma-Ray Burst Monitor detection: Yamada et al., GCN 20003) triggered Konus-Wind at T0=83274.317 s UT (23:07:54.317). The burst light curve shows a single pulse with a total duration of ~18 s. The emission is seen up to ~2 MeV. As observed by Konus-Wind, the burst had a fluence of 1.71(-0.07,+0.08)x10^-5 erg/cm2, and a 64-ms peak energy flux, measured from T0+4.144 s, of 3.43(-0.80,+0.80)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+15.616 s) is best fit in the 20 keV - 2 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.05(-0.09,+0.09), and Ep = 175(-12,+14) keV (chi2 = 56/58 dof). Fitting by the GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.14 (chi2 = 56/57 dof). The spectrum near the peak count rate (measured from T0 to T0+7.424 s) is best fit in the 20 keV - 2 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.71(-0.10,+0.11), and Ep = 210(-13,+15) keV (chi2 = 67/58 dof). Fitting by the GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.12 (chi2 = 67/57 dof). The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB161004_T83274/ All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.