TITLE: GCN CIRCULAR NUMBER: 23011 SUBJECT: Konus-Wind observation of GRB 180720B DATE: 18/07/22 13:43:39 GMT FROM: Dmitry Frederiks at Ioffe Institute D. Frederiks, S. Golenetskii, R.Aptekar, A. Kozlova, A.Lysenko, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long, extremely bright GRB 180720B (Swift-BAT detection: Siegel et al., GCN 22973, GCN 22975; Fermi-LAT detection: Bissaldi and Racusin, GCN 22980; Fermi-GBM detection: Roberts et al., GCN 22981) triggered Konus-Wind at T0=51705.261 s UT (14:21:45.261). The burst light curve shows a multi-peaked structure with a total duration (T100) of ~125 s. The emission is seen up to ~15 MeV. As observed by Konus-Wind, the burst had a fluence of (5.43 ± 0.26)x10^-4 erg/cm2 and a 64-ms peak energy flux, measured from T0+15.168, of (9.70 ± 0.52)x10^-5 erg/cm2 (both in the 20 keV - 10 MeV energy range). The time-integrated spectrum (measured from T0 to T0+119.808 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha = -1.01 (-0.06,+0.06), the high energy photon index beta = -2.07 (-0.08,+0.07), the peak energy Ep = 451 (-45,+52) keV, chi2 = 102/97 dof. The spectrum near the peak count rate (measured from T0+14.592 s to T0+15.360 s) is best fit in the 20 keV - 15 MeV range by the GRB (Band) function with the following model parameters: the low-energy photon index alpha = -0.82 (-0.09,+0.10), the high energy photon index beta = -2.27 (-0.22,+0.13), the peak energy Ep = 779 (-127,+169) keV, chi2 = 92/76 dof. Assuming the redshift z=0.654 (Vreeswijk et al., GCN 22996) and a standard cosmology model with H_0 = 70 km/s/Mpc, Omega_M = 0.30, and Omega_Lambda = 0.70, we estimate the following rest-frame parameters: the isotropic energy release E_iso is ~6.0x10^53 erg, the peak luminosity L_iso is ~1.8x10^53 erg/s, and the rest-frame peak energy of the time-integrated spectrum, Ep,z, is ~746 keV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB180720_T51705/ All the quoted errors are estimated at the 90% confidence level. All the presented results are preliminary.