GRB 250109A / EP250109a: Mephisto optical upper limits

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. -------------------------------------------------------------------------------------------------