TITLE: GCN CIRCULAR NUMBER: 40476 SUBJECT: GRB 250515A: 1.6m Mephisto optical detection DATE: 25/05/17 05:06:22 GMT FROM: zhjh@nao.cas.cn
Jinghua Zhang, Yu Pan, Xiangkun Liu, Yiheng Xie, Guowang Du, Yuan Fang, Xinlei Chen, Xingzhu Zou, Helong Guo, Xufeng Zhu, Tao Wang, Brajesh Kumar, Xinzhong Er, Yuanpei Yang, Xiaowei Liu (SWIFAR, YNU) report on behalf of Mephisto Team:
We conducted simultaneous multi-band photometric observations of the optical candidate of the likely short GRB 250515A (Fermi GBM team, GCN 40462) with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory. Mephisto was triggered ~17 hours post-alert and commenced observations starting from 13:47:01 UTC 2025-05-16 in moderate sky conditions. Multiple frames across uvgriz bands were acquired. The optical candidate (B. P. Gompertz et al., GCN 40464; Xin et al., GCN40474) was clearly detected in the stacked images of g and r bands while marginally detected in the u, v, i, z -bands. The preliminary photometry (AB magnitude system) are listed below:
| Time-Mid (2025-05-16 UT) | Filter | Exposure | Mag | Mag Err |
| 15:02:22 | u | 300sec*3 | 21.17 | 0.27 |
| 13:54:44 | v | 300sec*3 | 21.46 | 0.47 |
| 15:02:22 | g | 300sec*3 | 20.21 | 0.07 |
| 13:54:44 | r | 300sec*3 | 19.91 | 0.08 |
| 15:02:22 | i | 300sec*3 | 20.74 | 0.20 |
| 13:54:44 | z | 300sec*3 | 20.22 | 0.47 |
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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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