TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4394 SUBJECT: Konus-Wind observation of GRB 051221A DATE: 05/12/24 15:51:40 GMT FROM: Valentin Pal'shin at Ioffe Inst S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and T. Cline on behalf of the Konus-Wind team report: The GRB 051221A (Swift-BAT trigger #173780; Parsons et al., GCN 4363; Cummings et al., GCN 4365; Norris et al., GCN 4388) triggered Konus-Wind at T0=6672.976 s UT (01:51:12.976). The Konus-Wind light curve consists of a soft weak precursor and the main episode with five ~15-ms peaks. The first peak at T0-16 ms to T0+10 is substantially softer than the others (there is no emission in the 380-1160 keV energy range). After T0+0.250 sec a weak soft emission is marginally seen only in the G1 (18-70 keV) range up to ~1 sec . As observed by Konus-Wind the burst had a fluence of 3.2(-1.7, +0.1)x10^-6 erg/cm2 and peak flux on 4-ms time scale 4.6(-2.5, +0.2)x10^-5 erg/cm2/sec (both in the 20 keV - 2 MeV energy range). The time-integrated spectrum of the GRB (from T0 to T0+0.256 sec) is well fitted (in the 20 keV - 2 MeV range) by a power law with exponential cutoff model: dN/dE ~ E^(-alpha) * exp(-E/E0) with alpha = 1.08(-0.14, +0.13) and E0 = 436(-116, +165) keV (chi2 = 65/69dof). The peak energy Ep = 402(-72, +93) keV. The fitting by a single power law gives an unacceptable result: chi2 = 152/70dof (null hypothesis probability = 5.423E-08). All the quoted errors are at the 90% confidence level. Assuming z = 0.5465 (Berger and Soderberg, GCN 4384) and a standard cosmology model with H_0 = 70 km/s/Mpc, Omega_M = 0.3, Omega_\Lambda = 0.7, the isotropic energy release is E_iso ~2.5x10^51 erg, the maximum luminosity is (L_iso)_max ~5.5x10^52 erg/sec. The Konus-Wind light curve of this GRB can be seen at http://www.ioffe.rssi.ru/LEA/GRBs/GRB051221_T06672/