TITLE: GCN CIRCULAR NUMBER: 26576 SUBJECT: Konus-Wind observation of GRB 191221B DATE: 19/12/24 15:30:57 GMT FROM: Dmitry Frederiks at Ioffe Institute D. Frederiks, S. Golenetskii, R.Aptekar, A.Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long GRB 191221B (Swift detection: Laha et al., GCN 26534; AGILE/MCAL detection: Longo et al., GCN 26549; AstroSat CZTI detection: Gaikwad et al., GCN 26567) triggered Konus-Wind (KW) at T0=74349.647 s UT (20:39:09.647). The burst light curve shows a bright pulse with a complex, multi-peaked structure. The total duration of the burst is ~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/GRB191221_T74349/ As observed by Konus-Wind, the burst had a fluence of (1.0 ± 0.1)x10^-4 erg/cm^2 and a 64-ms peak energy flux, measured from T0+8.064, of (1.9 ± 0.2)x10^-5 erg/cm^2/s (both in the 20 keV - 10 MeV energy range). The time-integrated spectrum (measured from T0 to T0+21.760 s) is best fit in the 20 keV - 20 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha = -0.81 (-0.06,+0.06), the high energy photon index beta = -2.47 (-0.24,+0.16), the peak energy Ep = 377 (-29,+30) keV, chi2 = 99/97 dof. The spectrum near the peak count rate (measured from T0+6.912 s to T0+8.448 s) is best fit in the 20 keV - 20 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha = -0.51 (-0.11,+0.13), the high energy photon index beta = -2.72 (-0.50,+0.26), the peak energy Ep = 338 (-36,+38) keV, chi2 = 64/66 dof. Assuming the redshift z=1.148 (Vielfaure et al., GCN 26553) and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimate the following rest-frame parameters: the isotropic energy release E_iso = (3.6 ± 0.4)x10^53 erg, the peak luminosity L_iso = (1.5 ± 0.16)x10^53 erg/s, and the rest-frame peak energy of the time-integrated spectrum Ep,z=(810 ± 65) keV. With these values, GRB 191221B is within 68% prediction bands for both 'Amati' and 'Yonetoku' relations built for the sample of 138 long KW GRBs with known redshifts (Tsvetkova et al., ApJ 850 161, 2017), see http://www.ioffe.ru/LEA/GRBs/GRB191221_T74349/GRB191221B.pdf All the quoted errors are estimated at the 90% confidence level. All the presented results are preliminary.