TITLE: GCN CIRCULAR NUMBER: 38914 SUBJECT: EP250108a: Mephisto detection DATE: 25/01/12 03:05:54 GMT FROM: Chenxu Liu at Mephisto Team cxliu@ynu.edu.cn
Xingzhu Zou, Chenxu Liu, Brajesh Kumar, Xinlei Chen, Guowang Du, Tao Wang, Edoardo Lagioia, Yuan Fang, 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 X-ray transient EP250108a detected by EP-WXT (Li et al. GCN 38861) was confirmed with an optical counterpart by Eyles-Ferris (GCN 38878); Zhu et al., (GCN 38885); Malesani et al., (GCN 38902); Kumar et al., (GCN 38907) and L. Izzo (GCN 38861). The VLT/X-shooter spectroscopic analysis by Zhu et al. (GCN 38908) confirms the source redshift z = 0.176 and classified the transient as LFBOT (dubbed “the kangaroo”). We observed the field of EP250108a with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted on 2025-01-09 and 2025-01-10 (0.97 day and 2.06 days after the EP-WXT trigger) under fair observational conditions. Here, we report the preliminary analysis of our images taken on 2025-01-09. An optical counterpart is clearly detected (>5σ) at the transient location identified by Eyles-Ferris (GCN 38878) in the stacked images of uvgr bands. The preliminary u-, v-, g-, r- bands PSF photometry and the 3 sigma i-, z-bands upper limits are listed below. Further analysis is in progress and multi-band observations are planned in the upcoming nights.
Start_Time(UT) | Band | Exp | Mag/LimMag (AB) | Significance ------------------------|------|----------|------------------|-------------- 2025-01-09T11:45:52 | u | 180s*2 | 19.89 ± 0.23 | 6.4σ 2025-01-09T11:53:04 | v | 180s*2 | 20.02 ± 0.19 | 6.6σ 2025-01-09T11:45:53 | g | 50s*6 | 20.05 ± 0.14 | 7.8σ 2025-01-09T11:53:04 | r | 50s*6 | 20.45 ± 0.17 | 5.7σ 2025-01-09T11:45:53 | i | 79s*4 | >20.66 | < 3σ 2025-01-09T11:53:04 | z | 79s*4 | >19.92 | < 3σ ---------------------------------------------------------------------------------------- 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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