TITLE: GCN CIRCULAR NUMBER: 41982 SUBJECT: GRB 250924A: 1.6m Mephisto optical upper limits DATE: 25/09/25 11:07:51 GMT FROM: liuxk@ynu.edu.cn Helong Guo, Yichen Jin, Xingzhu Zou, Guowang Du, Xufeng Zhu, Brajesh Kumar, Xinlei Chen, Yuanpei Yang, Tao Wang, Xiangkun Liu and Xiaowei Liu (all SWIFAR, YNU) report on behalf of Mephisto Team: Simultaneous multi-band photometric observations of the GRB 250924A detected by Swift (Siegel et al., GCN 41959) was performed with 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. The observations were started from 16:00:12 2025-09-24 UT (~7.7 hr after the trigger) and six frames of 45 sec each were obtained in uvgr bands. The afterglow candidate (Becerra et. al., GCN 41960; Schneider et al., GCN 41963; Freeberg et. al., GCN 41968; Pankov et al., GCN 41969; Zhang & Filippenko, GCN 41970; Mo et al., GCN 41972, Siegel et al., GCN 41975, Ma et al., GCN 41976) is not detected in our uvgr stacked band images. The 3-sigma upper limit are listed below: ---------------------------------------------------------------------------- Start_Time(UT) Filter Exp (sec) LimMag(AB) 2025-09-24T16:00:15 u 45*6 >21.34 2025-09-24T16:06:46 v 45*6 >21.66 2025-09-24T16:00:12 g 45*6 >22.22 2025-09-24T16:06:43 r 45*6 >22.46 ---------------------------------------------------------------------------- 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/41982. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...