TITLE: GCN CIRCULAR NUMBER: 21852 SUBJECT: Konus-Wind observation of GRB 170904A DATE: 17/09/08 15:12:36 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, hard-spectrum, bright GRB 170904A (IPN Triangulation: Hurley et al., GCN 21820) triggered Konus-Wind at T0=35162.28 s UT (09:46:02.280). The burst light curve shows multiple emission pulses in the interval from ~T0-56 s ~T0+30 s. The emission is seen up to ~19 MeV. Weak count rate increases are seen in the same S1 KW detector at ~T0-560 s and ~T0-350 s, however their attribution to this GRB is unclear. As observed by Konus-Wind, the burst had a fluence of 1.73(-0.09,+0.09)x10^-4 erg/cm2, and a 64-ms peak energy flux, measured from T0+11.728 s, of 1.90(-0.32,+0.32)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+22.016 s) is best fit in the 20 keV - 19 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.69(-0.05,+0.06), the high energy photon index beta = -2.17(-0.10,+0.08), the peak energy Ep = 466(-36,+40) keV, chi2 = 115/97 dof. The spectrum near the peak count rate (measured from T0+9.984 to T0+11.776 s) is best fit in the 20 keV - 19 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.73(-0.11,+0.13), the high energy photon index beta = -2.19(-0.24,+0.15), the peak energy Ep = 441(-72,+87) keV, chi2 = 53/67 dof. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB170904_T35162/ All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.