TITLE: GCN CIRCULAR NUMBER: 38915 SUBJECT: GRB 250109A / EP250109a: Mephisto optical upper limits DATE: 25/01/12 04:25:54 GMT FROM: Chenxu Liu at Mephisto Team cxliu@ynu.edu.cn
Xingzhu Zou, Chenxu Liu, Guowang Du, Brajesh Kumar, Tao Wang, Edoardo Lagioia, Yuan Fang, Xinlei Chen, Yu Pan (all SWIFAR, YNU), Xuhui Han, Pinpin Zhang, Liping Xin, Chao Wu (all NAOC), Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:
The Mephisto team observed the field of the GRB 250109A / EP250109a (EP Team , GCN 38864, GCN 38889; Fermi GBM team, GCN 38873, GCN 38887; the Swift XRT team, GCN 38867, GCN 38899; James DeLaunay et al., GCN 38900; A. Moskvitin et al. GCN 38901; the SVOM team, GCN 38904) with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted starting from 12:27:02 2025-01-09 UT (6.15 hr after the EP-WXT trigger) under fair observational conditions. Several frames with different exposure time were taken. No new uncatalogued optical sources were detected within the EP-WXT error circle in our stacked images of uvgriz bands. The preliminary photometry and 3 sigma upper limits are below.
Start_Time(UT) | Band | Exp(s) | LimMag (AB) --------------------|------|--------|------------ 2025-01-09T12:27:02 | u | 180*2 | >20.02 2025-01-09T12:34:13 | v | 180*2 | >20.15 2025-01-09T12:27:03 | g | 50*6 | >20.64 2025-01-09T12:34:14 | r | 50*6 | >20.73 2025-01-09T12:27:02 | i | 79*4 | >20.14 2025-01-09T12:34:14 | z | 79*4 | >19.19
------------------------------------------------------------------------------------------------- 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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