TITLE: GCN CIRCULAR NUMBER: 39894 SUBJECT: GRB 250327B: 1.6m Mephisto optical observations DATE: 25/03/28 01:40:32 GMT FROM: Brajesh Kumar at SWIFAR, YNU brajesh@ynu.edu.cn

Jianhui Lian, Brajesh Kumar, Xufeng Zhu, Fanchuan Kong, Yaosong Yu, Yu Pan, Guowang Du, Xingzhu Zou, Yuan Fang, Xinzhong Er, Jinghua Zhang, Tao Wang, Xinlei Chen, Helong Guo, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:

The field of SVOM GRB 250327B (Bouchet et al., GCN 39888) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. 1 exposures of 180s in the MEPHISTO u, v, 3 exposures of 50s in the MEPHISTO g, r bands, and 2 exposures of 80s in the MEPHISTO i, z bands were obtained starting from 2025-03-27 T21:14:19 (~3min after the trigger). The afterglow candidate (Moskvitin et al., GCN 39889; Xin et al., GCN 39890; O’Neill et al., GCN 39891; Malesani et al., GCN 39893) is clearly detected in each v, g, r, i, z band, with v-r, g-i color of 3.99 and 1.80 mag, respectively. The preliminary magnitudes of the first detections are below:

Start_Time(UT) |Band | Exp(s) | Mag(AB) --------------------|-----|--------|---------------- 2025-03-27T21:18:21 | v | 180*1 | 18.66 +/- 0.075 2025-03-27T21:14:19 | g | 50*3 | 16.53 +/- 0.013 2025-03-27T21:18:21 | r | 50*3 | 14.67 +/- 0.003 2025-03-27T21:14:19 | i | 80*2 | 14.73 +/- 0.004 2025-03-27T21:18:21 | z | 50*3 | 13.83 +/- 0.004 ------------------------------------------------------------------------------------------------- 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. -------------------------------------------------------------------------------------------------

View this GCN Circular online at https://gcn.nasa.gov/circulars/39894. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...