TITLE: GCN CIRCULAR NUMBER: 26616 SUBJECT: GRB 191221B: CALET Gamma-Ray Burst Monitor detection DATE: 19/12/31 02:44:42 GMT FROM: Valentin Pal'shin at AGU S. Sugita, A. Yoshida, T. Sakamoto, V. Pal'shin (AGU), Y. Kawakubo (LSU), K. Yamaoka (Nagoya U), S. Nakahira (RIKEN), Y. Asaoka, S. Torii (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration: The long GRB 191221B (Swift detection: Barthelmy et al., GCN Circ. 26534, Sakamoto et al., GCN Circ. 26562; AGILE/MCAL observations: Longo et al., GCN Circ. 26549; AstroSat CZTI detection: Gaikwad et al., GCN Circ. 26567; Insight-HXMT/HE detection: Xue et al., GCN Circ. 26573; Konus-Wind observation: Frederiks et al., GCN Circ. 26576; https://gcn.gsfc.nasa.gov/other/191221B.gcn3) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 20:39:04.868 UTC on 21 December 2019. The burst signal was seen by all CGBM detectors. The burst light curve shows a multi-peaked pulse which starts at T+1.8 sec, peaks at T+11.3 sec and ends at T+23.4 sec. The T90 and T50 durations measured by the SGM data are 13.0 +- 1.6 sec and 4.4 +- 0.4 sec (40-1000 keV), respectively. The ground processed light curve will be available at http://cgbm.calet.jp/cgbm_trigger/ground/1260995788/ The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at the Waseda University.