TITLE: GCN CIRCULAR NUMBER: 29358 SUBJECT: Konus-Wind detection of GRB 210121A DATE: 21/01/26 16:18:39 GMT FROM: Dmitry Svinkin at Ioffe Institute D. Svinkin, S. Golenetskii, D. Frederiks, M. Ulanov, A. Tsvetkova, A. Lysenko, A. Ridnaia, and T. Cline on behalf of the Konus-Wind team, report: The long-duration, bright GRB 210121A (Insight-HXMT/HE detection: Xue et al., GCN Circ. 29346 GECAM detection: Peng et al., GCN Circ. 29347; IPN triangulation: Hurley et al., GCN Circ. 29348) triggered Konus-Wind at T0=67306.174 s UT (18:41:46.174). The burst light curve shows a multipeaked structure which starts at ~T0-0.1 s and has a total duration of ~64 s. The emission is seen up to ~7 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB210121_T67306/ As observed by Konus-Wind, the burst had a fluence of 1.18(-0.10,+0.10)x10^-4 erg/cm2, and a 64-ms peak flux, measured from T0+1.072 s, of 3.00(-0.43,+0.43)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+22.784 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.59(-0.05,+0.06), the high energy photon index beta = -3.03(-1.42,+0.43), the peak energy Ep = 875(-69,+70) keV (chi2 = 110/97 dof). The spectrum near the maximum count rate (measured from T0+0.256 s to T0+1.792 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.32(-0.08,+0.09), the high energy photon index beta = -3.60(3.60,+0.68), the peak energy Ep = 981(-88,+94) keV (chi2 = 82/97 dof). All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.