TITLE: GCN CIRCULAR NUMBER: 44233 SUBJECT: EP260402a / GRB 260402A: Mephisto optical upper limits DATE: 26/04/07 15:20:03 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 2026-04-02 19:52:10 UT (~9.4 hours after the Einstein Probe/WXT trigger; Zhao et al., GCN 44186) to observe the field of EP260402a. A set of simultaneous multi-band (u, g, v, r, i, z) images were collected with individual exposure times of 45 s per frame. Multiple exposures were obtained in each band and later stacked for analysis. The refined EP/FXT position (Zhao et al., GCN 44201) is fully covered by our field of view. Due to relatively poor seeing conditions during the observations (FWHM ~2.5–4.0 arcsec), the image quality is degraded compared to nominal performance, resulting in shallower limiting magnitudes. In our stacked images, no uncatalogued optical source is detected at the FXT position or within its error circle (radius ~10–20 arcsec). The corresponding 5-sigma limiting magnitudes (AB system, not corrected for Galactic extinction) are listed below: ---------------------------------------------------------------------------------------------- Start Time (UT) | Band | Exposure (s) | Mag / Limiting Mag ---------------------------------------------------------------------------------------------- 2026-04-02T19:52:10 | u | 45.0*2 | >19.77 2026-04-02T19:54:55 | v | 45.0*2 | >18.81 2026-04-02T19:52:23 | g | 45.0*2 | >20.21 2026-04-02T19:55:08 | r | 45.0*2 | >20.33 2026-04-02T19:52:13 | i | 45.0*2 | >20.23 2026-04-02T19:54:58 | z | 45.0*2 | >19.48 ---------------------------------------------------------------------------------------------- Our non-detection is consistent with previous optical and near-infrared upper limits reported by SVOM/COLIBRÍ (Antier et al., GCN 44183), MASTER (Carrasco et al., GCN 44184), Kilonova-Catcher (Freeberg et al., GCN 44185), SVOM/C-GFT (Wu et al., GCN 44189), SVOM/VT (Li et al., GCN 44190), WINTER (Schneider et al., GCN 44192), and Jinshan (He et al., GCN 44199). 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/44233. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...