TITLE: GCN CIRCULAR NUMBER: 39021 SUBJECT: GRB 250121A: CALET Gamma-Ray Burst Monitor detection DATE: 25/01/24 14:43:22 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University kawakubo@phys.aoyama.ac.jp

T. Sakamoto, A. Yoshida, S. Sugita, Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), 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), P. S. Marrocchesi (U of Siena), and the CALET collaboration:

The long GRB 250121A (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 39006; Fermi-LAT detection: Holzmann Airasca et al., GCN Circ. 39010; INTEGRAL/SPI-ACS detection: Barria et al., GCN Circ. 39011; Fermi GBM Observation: Godwin et al., GCN Circ. 39018; ) was detected in the ground analysis of the CALET Gamma-ray Burst Monitor (CGBM) data around 23:36:05.78 on 21 January 2025 (referenced to the Fermi-GBM Observation: GCN Circ. 39018). (https://cgbm.calet.jp/cgbm_trigger/flight/1421537655/index.html). The burst signal was seen by only the SGM detector.

The burst light curve shows a double-peaked structure that starts at T+0.1 sec, peaks at T+0.7 sec, and ends at T+5.4 sec. The T90 and T50 durations measured by the SGM data are 4.7 +/- 0.2 sec and 2.6 +/- 0.4 sec (40-1000 keV), respectively.

The ground-processed light curve is available at

https://cgbm.calet.jp/cgbm_trigger/ground/1421537655/

The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University.

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