TITLE: GCN CIRCULAR NUMBER: 38739 SUBJECT: GRB 241228B: Multiband optical monitoring with 1.6m Mephisto DATE: 24/12/31 09:02:12 GMT FROM: Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn> Brajesh Kumar, Yuan Fang, Jianhui Lian, Qinhao Shao, Bo Wu, Yu Pan, Xingzhu Zou, Xinlei Chen, Jinghua Zhang, Guowang Du, Yuanpei Yang, Yehao Cheng, Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team: The 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory was triggered to perform monitoring of GRB 241228B discovered by (Fermi GBM team, GCN 38682, Scotton and Meegan, GCN 38714) and Swift (Evans, GCN 38688; Burrows et al., GCN 38713). Simultaneous uvgriz band observations were started at 15:38:16 UT 2024-12-28 (~11.4 hr after the trigger) at an altitude of ~32 deg in moderate sky conditions (seeing ~2”). The GOTO optical counterpart GOTO24jmz (Kumar et al., GCN 38684, GCN 38691; An et al. GCN 38687; Ortega-Casas et al., GCN 38692; Ghosh et al., GCN 38702; An et al., GCN 38704; Volnova et al., GCN 38709; Strobl and Jelinek, GCN 38715, Moskvitin et al., GCN 38733) is clearly detected in different bands in the stacked images. The monitoring of the GRB field was continued at different epochs between 2024-12-28 to 2024-12-30 each night and multiple sets of observations in vri bands were acquired. The preliminary photometry on the stacked i-band initial frames is 20.2 +/- 0.1 mag (15:53:03UT2024-12-28, ExpStart). Further analysis of our remaining data is in progress. -------------------------------------------------------------------------- Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21. -------------------------------------------------------------------------- View this GCN Circular online at https://gcn.nasa.gov/circulars/38739. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...