TITLE: GCN CIRCULAR NUMBER: 37360 SUBJECT: GRB 240828B: Mephisto optical upper limit DATE: 24/08/31 13:25:50 GMT FROM: Brajesh Kumar at SWIFAR, YNU brajesh@ynu.edu.cn

Guowang Du (SWIFAR, YNU), Helong Guo (SWIFAR, YNU), Jinghua Zhang (SWIFAR, YNU), Tao Wang (SWIFAR, YNU), Zhenfei Qin (SWIFAR, YNU), Yicheng Jin (SWIFAR, YNU), Yu Pan (SWIFAR, YNU), Xingzhu Zou (SWIFAR, YNU), Xinlei Chen (SWIFAR, YNU), Yehao Cheng (SWIFAR, YNU), Yuan Fang (SWIFAR, YNU), Brajesh Kumar (SWIFAR, YNU), Xuhui Han (NAOC), Pinpin Zhang (NAOC), Liping Xin (NAOC), Chao Wu (NAOC), Yuanpei Yang (SWIFAR, YNU), Xinzhong Er (SWIFAR, YNU), Xiangkun Liu (SWIFAR, YNU), Xiaowei Liu (SWIFAR, YNU) report on behalf of Mephisto Team:

The field of SVOM/ECLAIRs-detected burst GRB 240828B (Le Provost et al., GCN 37318) was observed with 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous multi-band photometric observations were initiated at (UT) 19:25:20 Aug 28, (~6.21 hours after the burst notice). No new optical source was detected within the 2.6-arcsec Swift/XRT error circle (Page et al., GCN 37334). The 5-sigma limiting magnitudes in different bands are: u 20.75 (180 sec), v 20.97 (180 sec), g 20.54 (50 sec), r 20.83 (50 sec), i 20.44 (79 sec) and z 19.35 (79 sec). These are consistent with Zheng et al. (GCN 37345), Moskvitin et al. (GCN 37346), Wu et al. (GCN 37347), Xin et al. (GCN 37348), Wang et al. (GCN 37350), Schneider et al. (GCN 37352) and Akl et al. (GCN 37359).

------------------------------------------------------------------------------------------------------ Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6m 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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