TITLE: GCN CIRCULAR NUMBER: 44231 SUBJECT: EP260403a: Mephisto optical upper limits DATE: 26/04/07 15:11:25 GMT FROM: Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn> Chenxi Shang, Chenxu Liu, Guowang Du (SWIFAR, YNU), Jinghang Xue (NJU), Yuhui Zhang, Jialei Zheng, Tao Wang, Brajesh Kumar, Xueling Du, Xufeng Zhu, Yu Pan, Xingzhu Zou, Xinlei Chen, Yuan Fang, Jinghua Zhang (SWIFAR, YNU), Chao Wu (NAOC), Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team: The 1.6-m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory, was triggered at 2026-04-03 17:53:31 UT (~7.4 hours after the Einstein Probe/WXT trigger; Guo et al., GCN 44204) to observe the field of EP260403a. A set of simultaneous multi-band (u, v, i, z) images were collected with individual exposure times of 45 s per frame. The observations consist of multiple exposures in each band, which were later stacked for analysis. In the stacked images, we did not detect the optical counterpart reported by Antier et al. (GCN 44206), Pérez-Fournon et al. (GCN 44207) and Quirola-Vasquez (GCN 44207). Our non-detection is consistent with the continued fading of the optical source at later epochs. The estimated 5-sigma limiting magnitudes (AB system, not corrected for Galactic extinction) are listed below: ---------------------------------------------------------------- Start_Time(UT) | Band | Exp(s) | LimMag (AB) ---------------------------------------------------------------- 2026-04-03T17:53:31 | u | 45.0*2 | >19.62 2026-04-03T17:55:57 | v | 45.0*2 | >18.79 2026-04-03T17:53:33 | i | 45.0*2 | >19.72 2026-04-03T17:56:00 | z | 45.0*2 | >19.16 ---------------------------------------------------------------- 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. The facility is operated by the South-Western Institute for Astronomy Research (SWIFAR), Yunnan University. It provides real-time, high-quality colors of stellar objects. The Mephisto mosaic cameras were installed in October 2025. The first light was achieved in all three channels on 10 October 2025 and presently, these are under the commissioning phase. All the data have been reduced by the Mephisto data processing pipeline. We note that the current data-processing pipeline is still at a preliminary stage, with flux calibration precision of about 2% in the u and v bands, about 1% in the g and r bands, and better than 1% in the i and z bands. View this GCN Circular online at https://gcn.nasa.gov/circulars/44231. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...