TITLE: GCN CIRCULAR NUMBER: 43250 SUBJECT: GRB 251222A/EP251222b: 1.6m Mephisto optical followup DATE: 25/12/29 11:31:46 GMT FROM: Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn> Brajesh Kumar, Shiyan Zhong, Guowang Du, Hongguang Li, Dan Zhu, Yu Pan, Yuan Fang, Xingzhu Zou, Xinlei Chen, Xufeng Zhu, Tao Wang, Jinghua Zhang, Yehao Cheng, Xinzhong Er, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team: We observed GRB 251222A/EP251222b (SVOM burst-id sb25122206, Yang et al. GCN 43188; Fermi/GBM, GCN 43186; EP-WXT, Guo et al., GCN 43201) with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at the Lijiang Observatory. Multi-band (ugi and vrz) simultaneous observations were acquired in two sets using the newly installed three mosaic cameras. The first set (in ugi bands) and the second set (in vrz and ugi bands) of observations were started from 17:11:08T2025-12-22 (~5 min after the SVOM trigger) and 17:48:08T2025-12-22, respectively. These observations were continued till 2025-12-22T19:23:19. The optical counterpart reported previously (Palmerio et al., GCN 43189; An et. al, GCN 43194; Calapai et. al, GCN 43195; Durbak et. al, GCN 43197; Li et. al, GCN 43203; Saccardi et. al, GCN 43208; Gupta et. al, GCN 43207 GCN 43224; Saccardi et. al, GCN 43204; Kumar et. al, GCN 43209; Pérez-Fournon et. al, GCN 43212; Globus et al., GCN 43225; Pankov et al., GCN 43226; Freeberg et al., GCN 43229; Volnova et al., GCN 43230; Busmann et al., GCN 43233; Lee et al., GCN 43236; Maksut et al., GCN 43237) was clearly detected in our vgriz bands but not in u band. Although our first set of observations is limited, the overall preliminary light curves indicate a decay trend during ~300 - 2500 sec and a rise afterward. The preliminary photometric magnitudes of initial frames are listed below. These magnitudes are not corrected for the Galactic extinction in the direction of the burst. ------------------------------------------------------------------ Start_Time(UT) |Band | Exp(s)| Mag/LimMag (AB) ---------------------|------|-------|----------------------------- 2025-12-22T17:11:08 | u | 45*2 | >20.85 2025-12-22T17:48:08 | v | 60*3 | 21.41 +/- 0.32 2025-12-22T17:11:11 | g | 45 | 18.28 +/- 0.05 2025-12-22T17:48:10 | r | 60 | 19.18 +/- 0.08 2025-12-22T17:11:11 | i | 45 | 17.60 +/- 0.03 2025-12-22T17:48:10 | z | 60 | 19.02 +/- 0.23 ------------------------------------------------------------------ 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. Here, we note that the current data processing pipeline is still at a preliminary stage, with flux calibration precision in each band at the level of about 5% or even higher. ------------------------------------------------------------------- View this GCN Circular online at https://gcn.nasa.gov/circulars/43250. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...