TITLE: GCN CIRCULAR NUMBER: 18810 SUBJECT: GRB 151231A: CALET Gamma-Ray Burst Monitor detection DATE: 16/01/03 08:07:31 GMT FROM: Takanori Sakamoto at AGU M. Moriyama, A. Yoshida, T. Sakamoto, I. Takahashi, Y. Kawakubo, K. Senuma, Y. Yamada (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka, S. Ozawa, S. Torii (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena) and the CALET collaboration: The long-duration GRB 151231A (Veres et al. GCN Circ. 18787) triggered the CALET Gamma-Ray Burst Monitor (CGBM) at 10:37:53.15 UT on 31 December 2015. The clear burst signal was detected by all instruments. The light curve of the SGM shows two main peaks. The first peak starts at T0-2 sec, peaks at T0 and ends at T0+5 sec. The second peak starts at T0+50 sec, peaks at T0+60 sec and ends at T0+70 sec. The T90 duration measured by the SGM data is 66.0 +- 0.7 sec (40-1000 keV). The CGBM data used in this analysis are provided by the Waseda CALET Operation Center located at the Waseda University.