TITLE: GCN CIRCULAR NUMBER: 39929 SUBJECT: GRB 250329B: 1.6m Mephisto Optical Upper Limits DATE: 25/03/29 18:00:42 GMT FROM: Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn> Xinlei Chen, Brajesh Kumar, Jianhui Lian, Xufeng Zhu, Fanchuan Kong, Yaosong Yu, Yu Pan, Xingzhu Zou, Guowang Du, Yuan Fang, Jinghua Zhang, Tao Wang, Helong Guo, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team: We observed the field of SVOM/ECLAIRs-detected GRB 250329B (Xie et al., GCN 39925) with 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band observations were started from 2025-03-29 UT13:29:03 (~1.3 hr after the trigger) and multiple frames with different exposure times were taken under moderate sky conditions. In our stacked images, no new optical source was detected, consistent with Xin et al. (GCN 39926). The 5-sigma limiting magnitudes in different bands are below: Start_Time(UT) | Band | Exp(s) | LimMag(AB) ---------------------|------|--------------|------------ 2025-03-29T13:29:05 | u | 180*2, 300*2 | >19.93 2025-03-29T13:43:31 | v | 180*1, 300*2 | >19.76 2025-03-29T13:29:04 | g | 50*6, 300*2 | >20.58 2025-03-29T13:43:30 | r | 50*3, 300*2 | >20.55 2025-03-29T13:29:03 | i | 79*4, 300*2 | >20.68 2025-03-29T13:43:30 | z | 79*2, 300*2 | >19.65 ------------------------------------------------------------------------------------------------- Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m 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/39929. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...