TITLE: GCN CIRCULAR NUMBER: 43947 SUBJECT: GRB 260307B: Mephisto optical upper limits DATE: 26/03/08 13:15:21 GMT FROM: Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn> Guowang Du, Weikang Lin, Xueling Du, Donglin Gao, Xueteng Zhang, Yu Pan, Xingzhu Zou, Xinlei Chen, Brajesh Kumar, Yuan Fang, Jinghua Zhang, Xufeng Zhu, Tao Wang, Xinzhong Er, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team: The 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory, was triggered at 2026-03-07T13:48:27 (~181 seconds after the SVOM/ECLAIRs trigger) to observe the field of SVOM GRB 260307B (Magnani et al., GCN 43940). A set of simultaneous multi-band (ugi and vrz) images was collected. In our stacked frames, no optical candidate was detected within the SVOM/ECLAIRs localization and at the position reported by Corcoran et al. (GCN 43946). The upper limits (3 sigma) are listed below, which are consistent with the non-detection previously reported in Kang. et al., GCN 43941, Palmerio. et al., GCN 43942, Vijaykumar et al., GCN 43943 and Ma et al., GCN 43945. ----------------------------------------------------------------------------- Start_Time(UT) | Band | Exp(s) | LimMag (AB) ----------------------------------------------------------------------------- 2026-03-07T13:48:27 | u | 45.0*7,300.0*2 | >21.67 2026-03-07T13:51:28 | v | 45.0*8,300.0*2 | >21.94 2026-03-07T13:48:29 | g | 45.0*6,300.0*2 | >22.23 2026-03-07T13:51:30 | r | 45.0*8,300.0*2 | >22.54 2026-03-07T13:48:29 | i | 45.0*6,300.0*2 | >22.15 2026-03-07T13:51:30 | z | 45.0*8,300.0*2 | >21.31 ----------------------------------------------------------------------------- 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 ~2% in the u and v bands, ~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/43947. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...