TITLE: GCN CIRCULAR NUMBER: 40373 SUBJECT: GRB 250506A: Mephisto optical upper limits DATE: 25/05/07 02:43:57 GMT FROM: Brajesh Kumar at SWIFAR, YNU brajesh@ynu.edu.cn
Dezi Liu, Ziwei Li, Tao Wang, Shuang Lin, Xiangru Qian, Guowang Du, Xingzhu Zou, Yu Pan, Xinlei Chen, Xufeng Zhu, Helong Guo, Yuan Fang, Jinghua Zhang, Chenxu Liu, Edoardo Lagioia, Brajesh Kumar, Jianhui Lian, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:
We conducted simultaneous multi-band photometric observations of the GRB 250506A (SVOM burst-id sb25050601) detected by SVOM/ECLAIRs (Wang et al., GCN 40358) and by Fermi (Fermi GBM team, GCN 40362) with the Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory. Simultaneous u, g, and i band observations were initiated at 14:08:39 UTC on 2025-05-06 (~12.5 hours after the trigger). We obtained four exposures of 300 seconds each in ugi bands. No optical counterpart was detected at the refined position reported by the SVOM team (Maggi et al., GCN 40364). The preliminary 3-sigma upper limits are below:
| UT Start | T-T0 (hr) | Band | Exp | LimMag (AB) | |-------------------|-----------|------|--------|-------------| | 2025/5/6 14:08:40 | 12.53 | u | 4x300s | > 21.82 | | 2025/5/6 14:08:40 | 12.53 | g | 4x300s | > 22.42 | | 2025/5/6 14:08:39 | 12.53 | i | 4x300s | > 21.84 |
These limits are consistent with previously reported non-detections (Xin et al., GCN 40366; Ferro et al., GCN 40363; Liu et al., GCN 40360; Postigo et al., GCN 40359).
---------------------------------------------------------- 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. ----------------------------------------------------------
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