TITLE: GCN CIRCULAR NUMBER: 37582 SUBJECT: EP240913a/GRB 240913C: 1.6m Mephisto observations DATE: 24/09/21 03:16:25 GMT FROM: Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn> Xingzhu Zou, Xiangkun Liu, Yangwei Zhang, Xin Chang, Yaosong Yu, Jinghua Zhang, Yuan Fang, Guowang Du, Yu Pan, Xinlei Chen, Brajesh Kumar, Yuanpei Yang, Xinzhong Er and Xiaowei Liu (SWIFAR, YNU) report on behalf of Mephisto Team: We observed the fast X-ray transient EP240913a (Li et al., GCN 37492, which has been proposed to be a GRB event by Yin et al., GCN 37493) with 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Multiple frames in ugi and vrz bands were simultaneously acquired on 2024-09-14 starting from 14:35:17 (~26.9 hrs after EP T0). In the co-added frames, no evidence of any new uncataloged optical source was detected within the WXT error circle of 2.5 arcmin reported by Li et al. (GCN 37492). Existing sources within this error circle exhibited no significant optical enhancement. The table below provides details of the observations and the measured 3-sigma upper limits on co-added frames (in the AB magnitude system) taken with Mephisto. This non-detection result of the transient is in agreement with the previous circulars (Asquini et al., GCN 37499, Zhu et al., GCN 37500, 37501, Zheng et al., 37511, Schneider et al., GCN 37515, Lipunov et al., GCN 37521). Time-start Time-end Filter Exposure U.L.(AB) 14:35:17 14:44:46 u 180s*3 21.29 14:46:15 14:52:29 v 180s*2 20.87 14:35:18 14:44:45 g 50s*9 21.90 14:46:16 14:52:26 r 50s*6 21.42 14:35:17 14:44:48 i 79s*6 21.25 14:49:30 14:52:28 z 79s*4 19.52 ------------------------------------------------------------------------------------------------------ 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/37582. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...