TITLE: GCN CIRCULAR NUMBER: 40058 SUBJECT: EP250404a: Mephisto multi-band optical detection DATE: 25/04/04 16:19:08 GMT FROM: Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn> Guowang Du, Ziwei Li, Chenxu Liu, Dezi Liu, Yaosong Yu, Chenxi Shang, Jinghua Zhang, Yuan Fang, Xufeng Zhu, Tao Wang, Xinlei Chen, Xingzhu Zou, Yu Pan, Brajesh Kumar, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team: We performed simultaneous multi-band photometric observations of EP250404a (EP-WXT ID: 01709133918) detected by EP (Hu et al., GCN 40051) and by Fermi/GBM (Fermi team, GCN 40050) with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at the Lijiang Observatory. The observations were initiated at 14:32:03UTC2025-04-04 (i.e., 723s after the EP/WXT trigger) and are still ongoing at the time of this report. Multiple frames were obtained in uvgriz- bands. The optical counterpart is clearly detected in all bands. The photometry of the initial frames is listed below: UT start |Band | Exp | Mag (AB) --------------------|-----|-------|----------|-------------------------- 2025/4/4 14:32:03 | u | 180s | 18.65+/-0.08 2025/4/4 14:32:04 | i | 79s | 14.60+/-0.01 2025/4/4 14:32:05 | g | 50s | 15.72+/-0.01 2025/4/4 14:39:20 | v | 180s | 16.91+/-0.02 2025/4/4 14:39:21 | z | 79s | 14.03+/-0.01 2025/4/4 14:39:22 | r | 50s | 14.83+/-0.01 Our results echo the detection from Nanshan/HMT (Jiang et al., GCN 40052) and BOOTES-4/MET (I. Perez-Garcia et al., GCN 40053). ---------------------------------------------------------- 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/40058. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...