TITLE: GCN CIRCULAR NUMBER: 37568 SUBJECT: EP240918a: 1.6m Mephisto multi-band Optical Upper Limit DATE: 24/09/20 12:21:59 GMT FROM: Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn> Jinghua Zhang (SWIFAR, YNU), Xingzhu Zou (SWIFAR, YNU), Guowang Du (SWIFAR, YNU), Tao Wang (SWIFAR, YNU), Jianhui Lian (SWIFAR, YNU), Jiang Runnan (SWIFAR, YNU), Yu Pan (SWIFAR, YNU), Xinlei Chen (SWIFAR, YNU), Yuan Fang (SWIFAR, YNU), Brajesh Kumar (SWIFAR, YNU), Xinzhong Er (SWIFAR, YNU), Xiangkun Liu (SWIFAR, YNU), Xiaowei Liu (SWIFAR, YNU) report on behalf of Mephisto Team: Simultaneous multi-band photometric observations of the fast X-ray transient EP240918a (Zhang et al., GCN 37541) were conducted using the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) and the Twin 50-cm Telescopes at Yunnan University's Lijiang Observatory. Mephisto was triggered ~2.33 hours post-alert and commenced observations at 13:43:57 UTC on 2024-09-18 under moderate sky conditions, capturing multiple frames across uvgriz bands. Similarly, the two 50-cm Telescopes were activated 2.25 hours after the notice, beginning observations at 13:39:07 UTC, and collected frames in gr bands. No evidence of a new, uncataloged optical source was detected within the FXT error circle of 20" reported by Zhang et al. (GCN 37541). Existing sources within this error circle exhibited no significant optical enhancement. Details of the observations and the measured 3-sigma upper limits (in the AB magnitude system) with Mephisto and 50-cm are listed below: Time-start Time-end Telescope Filter Exposure U.L.(AB) 14:17:50 14:20:50 Mephisto u 180s*1 20.16 13:43:58 14:33:02 Mephisto v 180s*7 22.05 14:17:51 14:20:51 Mephisto g 180s*1 20.85 13:43:57 14:32:59 Mephisto r 50s*24 22.79 14:17:51 14:20:51 Mephisto i 180s*1 20.61 13:44:02 14:33:05 Mephisto z 79s*15 21.01 13:39:07 13:55:33 50-cm g 300s*4 21.69 13:39:09 13:55:33 50-cm r 300s*4 21.69 The non-detection result of the transient is in agreement with the previous circulars (Wang et al., GCN 37544, Kong et al., GCN 37545, Sun et al., GCN 37546, Fu et al., GCN 37547, Lipunov et al., GCN 37548). ------------------------------------------------------------------------------------------------------ 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/37568. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...