TITLE: GCN CIRCULAR NUMBER: 18854 SUBJECT: Konus-Wind observation of GRB 151229B DATE: 16/01/12 16:33:23 GMT FROM: Anastasia Tsvetkova at Ioffe Institute S. Golenetskii, R. Aptekar, D. Frederiks, P. Oleynik, M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, A. Kozlova, and T. Cline on behalf of the Konus-Wind team, report: The long-duration, extremely intense GRB 151229B (IPN triangulation: Golenetskii et al., GCN Circ. 18852) triggered Konus-Wind at T0=77056.981 s UT (21:24:16.981). The burst light curve shows a bright multi-peaked pulse with a duration of ~10 s (from ~T0+5 s to ~T0+15 s). This extremely bright phase of the event passes into a weaker decaying tail, which can be traced out to ~T0+100 s. The emission in the main pulse is seen up to ~15 MeV. As observed by Konus-Wind, the burst had a fluence of 4.2(-0.1,+0.1)x10^-4 erg/cm2, and a 64-ms peak flux, measured from T0+10.064 s, of 7.1(-0.6,+0.6)x10^-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+28.672 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.65(-0.06,+0.06), the high energy photon index beta = -2.12(-0.03,+0.03), the peak energy 262(-13,+14) keV (chi2 = 120/93 dof) The spectrum near the maximum count rate (measured from T0+9.984 to T0+10.240 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.33(-0.14,+0.16), the high energy photon index beta = -2.51(-0.18,+0.14), the peak energy 290(-30,+33) keV (chi2 = 44/54 dof) The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB151229_T77056/ All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.