TITLE: GCN CIRCULAR NUMBER: 31719 SUBJECT: CALET Gamma-Ray Burst Monitor detection of GRB 220308A and its possible precursor DATE: 22/03/09 09:01:19 GMT FROM: Valentin Pal'shin at AGU P. S. Marrocchesi (U of Siena), A. Yoshida, T. Sakamoto, V. Pal'shin, S. Sugita (AGU), Y. Kawakubo (LSU), K. Yamaoka (Nagoya U), S. Nakahira (RIKEN), Y. Asaoka (ICRR), S. Torii, Y. Akaike, K. Kobayashi (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence), and the CALET collaboration: At 05:35:27.809 UTC on 8 March 2022, the CALET Gamma-ray Burst Monitor (CGBM) triggered on a long weak GRB followed in ~460 s by the bright GRB 220308A, localized by the IPN (Svinkin et al., GCN 31715; https://gcn.gsfc.nasa.gov/other/220308A.gcn3). Both bursts were seen by all CGBM detectors. The first (trigger) burst might be a precursor of GRB 220308A. The trigger burst light curve shows a double-peaked pulse which starts at T+1.9 sec, peaks at T+3.6 sec,and ends at T+15.0 sec. The T90 and T50 durations measured by the SGM data are 11.6 +- 1.6 sec and 7.4 +- 1.6 sec (40-1000 keV), respectively. The GRB 220308A light curve shows a multi-peaked structure which starts at T+461.6 sec, peaks at T+465.9 sec, and ends at T+481.9 sec. The T90 and T50 durations measured by the SGM data are 15.4 +- 0.4 sec and 4.0 +- 0.1 sec (40-1000 keV), respectively. The ground processed light curve is available at http://cgbm.calet.jp/cgbm_trigger/ground/1330752907/ The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at the Waseda University.