Mephisto optical upper limit of EP250321b

ATel #17106; Kaushik Chatterjee, Weikang Lin, Tao Wang, Haipeng Lei (SWIFAR, YNU), Litao Zhu (Dept. of Astro., YNU), Yu Pan, Guowang Du, Xingzhu Zou, Brajesh Kumar, Yuan Fang, Helong Guo, Edoardo P. Lagioia, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of Mephisto team:
on 24 Mar 2025; 17:00 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Brajesh Kumar (brajesh@ynu.edu.cn)

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The field of EP250321b (Cheng et al., ATel #17103) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. 3 exposures of 300s in the MEPHISTO u, v, g, r bands, and 6 exposures of 300s in the MEPHISTO i band were obtained starting from 2025-03-23 T20:58:38 (~1.9d after the trigger). In our stacked images, we did not detect any source at the position reported by Cheng et al., ATel #17103. The preliminary 3-sigma upper limits are below: Start_Time(UT) 2025-03-23T20:58:38, u, 300*3 > 21.26; 2025-03-23T20:58:38, g, 300*3 > 20.51; 2025-03-23T20:58:38, i, 300*6 > 21.43; 2025-03-23T21:20:32, v, 300*3 > 22.07; 2025-03-23T21:20:31, r, 300*3 > 21.50. 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.