TITLE: GCN CIRCULAR NUMBER: 41631 SUBJECT: EP250901a: Mephisto optical upper limits DATE: 25/09/01 18:00:45 GMT FROM: liuxk@ynu.edu.cn Xiangkun Liu, Chenxu Liu, Guowang Du, Xufeng Zhu, Shuai Xie, Xiaowei Liu (all SWIFAR, YNU), and Chao Wu (EP team) report on behalf of the Mephisto Team: The field of EP250901a, detected by EP-WXT (ID: 00109201113, triggered at 2025-09-01T12:05:03 UTC) and subsequently followed up by EP-FXT (GCN 41627), was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgr band photometric observations were conducted starting from 13:30:37 2025-09-01 UT (~1.5 hr after the trigger) and 6 frames with 45s exposure times were taken. No new optical counterpart was detected (consistent with the upper limits reported by Global MASTER-Net, GCN 41629) in the stacked images of uvgr bands. The preliminary photometry and 3 sigma upper limits are listed below. Start_Time(UT) | Band | Exp(s) | LimMag (AB) --------------------|------|--------|------------ 2025-09-01T13:30:37 | u | 45*6 | >21.68 2025-09-01T13:30:37 | v | 45*6 | >21.77 2025-09-01T13:37:13 | g | 45*6 | >22.17 2025-09-01T13:37:13 | r | 45*6 | >22.31 ------------------------------------------------------------------------------------------------- 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/41631. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...