TITLE: GCN CIRCULAR NUMBER: 19604 SUBJECT: Konus-Wind observation of GRB 160625B DATE: 16/06/27 16:30:09 GMT FROM: Dmitry Frederiks at Ioffe Institute D. Svinkin, S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik, M. Ulanov, A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline on behalf of the Konus-Wind team, report: The long-duration, extremely bright GRB 160625B (Fermi-LAT detection: Dirirsa et al., GCN Circ. 19580; Fermi-GBM detection: Burns, GCN Circ. 19581) triggered Konus-Wind at T0=81619.875 s UT (22:40:19.875). The burst light curve starts, at ~T0-0.3 s, with a short soft initial pulse (precursor) which had a duration of about 1 s. The precursor is followed, starting at ~T0+180 s, by the main, extremely bright and spectrally-hard episode with a duration of about 40 s. This episode evolves into a weak emission traceable up to ~T0+680 s. The emission in the main episode is seen up to ~20 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB160625_T81619/ As observed by Konus-Wind, the burst had a fluence of 9.44(-0.16,+0.16)x10^-4 erg/cm2, and a 256-ms peak flux, measured from T0+188.928 s, of 1.26(-0.10,+0.10)x10^-4 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the initial pulse (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 1 MeV range by a simple power law model with the photon index alpha = -2.34(-0.22,+0.25) (chi2 = 38/61 dof) The time-averaged spectrum of the main episode (measured from T0+180.480 to T0+229.632 s) is best fit in the 20 keV - 20 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.79(-0.02,+0.02), the high energy photon index beta = -2.25(-0.04,+0.04), the peak energy Ep = 554(-19,+20) keV (chi2 = 353/97 dof) The spectrum near the maximum count rate (measured from T0+188.928 to T0+189.184 s) is best fit in the 20 keV - 20 MeV range by the Band model with the following parameters: the low-energy photon index alpha = -0.77(-0.08,+0.10), the high energy photon index beta = -2.22(-0.25,+0.17), the peak energy Ep = 937(-179,+195) keV (chi2 = 62/67 dof) Assuming the redshift z=1.406 (Xu, et al., GCN Circ. 19600) and a standard cosmology model with H_0 = 70 km/s/Mpc, Omega_M = 0.27, and Omega_Lambda = 0.73, we estimate the following rest-frame parameters: the isotropic energy release E_iso is ~5.0x10^54 erg, the peak luminosity L_iso is ~1.6x10^54 erg/s, and the rest-frame peak energy of the time-integrated spectrum, Ep,i, is ~1.3 MeV. This Eiso is the highest one measured by Konus-Wind for more than 21 years of its GRB observations and, probably, the highest Eiso recorded for a GRB ever. All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.