TITLE: GCN CIRCULAR NUMBER: 40197 SUBJECT: EP250421a: Mephisto optical counterpart confirmation DATE: 25/04/22 04:28:53 GMT FROM: liuxk@ynu.edu.cn Xiangkun Liu, Xinlei Chen, Yi Zhang, Runnan Jiang, Brajesh Kumar, Guowang Du, Yuan Fang, Jinghua Zhang, Xingzhu Zou, Yu Pan, Yuanpei Yang, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team: The field of EP250421a (EP-WXT trigger ID: 01709135179) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted starting from 16:26:55 2025-04-21 UT (~5 mins after the trigger) and 7 frames with 45s exposure time for ugi bands and 6 frames with 45s exposure times for vrz bands were taken, followed by another 2 frames with 300s exposure time for all ugi/vrz bands. We detected an optical transient candidate (Lee et al., GCN 40192; Fu et al., GCN 40193) in the stacked riz band images in both sets of observations but no detection in uvg bands. In our priliminary photometry there is clear indication of the decay of the source as listed below. Start_Time(UT) | Band | Exp(s) | Mag/LimMag (AB) ---------------------------|--------|---------|------------ 2025-04-21T16:26:55 | u | 45*7 | >21.15 2025-04-21T16:38:05 | v | 45*6 | >21.39 2025-04-21T16:26:55 | g | 45*7 | >22.02 2025-04-21T16:38:05 | r | 45*6 | 20.47 +/- 0.10 2025-04-21T16:26:55 | i | 45*7 | 19.07 +/- 0.07 2025-04-21T16:38:05 | z | 45*6 | 18.62 +/- 0.13 Start_Time(UT) | Band | Exp(s) | Mag/LimMag (AB) ---------------------------|--------|---------|------------ 2025-04-21T17:12:15 | u | 300*2 | >21.41 2025-04-21T16:59:53 | v | 300*2 | >21.64 2025-04-21T17:12:15 | g | 300*2 | >22.27 2025-04-21T16:59:53 | r | 300*2 | 21.21 +/- 0.16 2025-04-21T17:12:15 | i | 300*2 | 20.28 +/- 0.12 2025-04-21T16:59:53 | z | 300*2 | 19.15 +/- 0.14 ------------------------------------------------------------------------------------------------- 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/40197. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...