TITLE: GCN CIRCULAR NUMBER: 39946 SUBJECT: GRB 250328A: 1.6m Mephisto optical observations DATE: 25/03/30 16:19:25 GMT FROM: Brajesh Kumar at SWIFAR, YNU brajesh@ynu.edu.cn
Brajesh Kumar, Jianhui Lian, Xinlei Chen, Xufeng Zhu, Fanchuan Kong, Yaosong Yu, Xingzhu Zou, Yu Pan, Guowang Du, Yuan Fang, Jinghua Zhang, Helong Guo, Tao Wang, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:
The field of SVOM/ECLAIRs-detected GRB 250328A (burst-id sb25032803; Brunet et al., GCN 39910) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Our observations were started from 2025-03-28T18:19:29 (~1.4 hr after the trigger) under moderate sky conditions. Multiple frames in uvgriz bands were simultaneously (ugi, vrz) acquired with different exposure time. There is a marginal detection of an afterglow candidate in our stacked g and r band images at the location reported in VLT/HAWK-I observations (Schneider et al.; GCN 39919), also consistent with the XRT source #2. There is no optical detection in uviz bands. Considering the fading nature of the source reported in later epoch of observations at the same location (Schneider et al., GCN 39920; Li et al., GCN 39923; Li et al, GCN 39936), the candidate in our stacked images is likely the optical counterpart of GRB 250328A (Li et al, GCN 39936). The preliminary photometric magnitudes (without Galactic extinction correction) and 3 sigma limiting magnitudes in different bands are below:
Start_Time(UT) | Band | Exp(s) | Mag/LimMag(AB) ---------------------|------|--------|-------------------- 2025-03-28T18:19:29 | u | 180*3 | >20.17 2025-03-28T18:28:23 | v | 180*4 | >21.02 2025-03-28T18:19:29 | g | 50*10 | 21.53 +/- 0.22 2025-03-28T18:28:23 | r | 50*12 | 21.44 +/- 0.15 2025-03-28T18:19:29 | i | 79*7 | >21.62 2025-03-28T18:28:23 | z | 79*8 | >20.94 ---------------------------------------------------------- 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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