TITLE: GCN CIRCULAR NUMBER: 15670 SUBJECT: Konus-Wind observation of GRB 131231A DATE: 14/01/03 17:15:47 GMT FROM: Dmitry Frederiks at Ioffe Institute S. Golenetskii, R.Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf of the Konus-Wind team, report: The long-duration GRB 131231A (Fermi-LAT detection: Sonbas, et al., GCN 15640; Fermi-GBM observation: Jenke & Xiong, GCN 15644) triggered Konus-Wind at T0=17132.361 s UT (04:45:32.361). The light curve shows a broad mult-peaked pulse from ~T0-13 s to ~T0+35 s. The emission is seen up to ~15 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB131231_T17132/ Note: periodic 'dips' in the count rate are due to the GRB source occultation by the s/c structure. As observed by Konus-Wind, the burst had a fluence of (1.55 ± 0.05)x10-4 erg/cm2, and a 64-ms peak flux, measured from T0+7.488 s, of (2.14 ± 0.13)x10-5 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum (measured from T0 to T0+34.303 s) is best fit in the 35 keV - 18 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha = -1.28 ± 0.04, the high energy photon index beta = -2.47 ± 0.05, the peak energy Ep = 163 ± 6 keV, chi2 = 93.8/95 dof. The spectrum near the maximum count rate (measured from T0+5.888 s to T0+8.448 s) is best fit in the 35 keV - 10 MeV range by the cutoff power law with the following model parameters: the photon index alpha = -0.76 ± 0.09, and the peak energy Ep = 226 ± 14 keV, chi2 = 94.3/82 dof. Assuming the redshift z=0.644 (Xu, et al., GCN 15645; Cucchiara, GCN 15652), 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 (1.7 ± 0.1)x10^53 erg, the peak luminosity L_iso is (3.9 ± 0.2)x10^52 erg/s, and the rest-frame peak energy Ep,i = (270 ± 10) keV All the quoted results are preliminary.