TITLE: GCN CIRCULAR NUMBER: 38183 SUBJECT: GRB 241112B: Optical afterglow detection confirmation in revisiting the Mephisto data DATE: 24/11/12 20:22:30 GMT FROM: Brajesh Kumar at SWIFAR, YNU brajesh@ynu.edu.cn

Guowang Du (SWIFAR, YNU), Weikang Lin (SWIFAR, YNU), Brajesh Kumar (SWIFAR, YNU), Yaosong Yu (SWIFAR, YNU), Yehao Cheng (SWIFAR, YNU), Xinlei Chen (SWIFAR, YNU), Yu Pan (SWIFAR, YNU), Xingzhu Zou (SWIFAR, YNU), Jinghua Zhang (SWIFAR, YNU), Yuanpei Yang (SWIFAR, YNU), Yuan Fang (SWIFAR, YNU), Xuhui Han (NAOC), Pinpin Zhang (NAOC), Liping Xin (NAOC), Chao Wu (NAOC), Xiangkun Liu (SWIFAR, YNU), Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team:

The 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory was triggered to observe GRB 241112B (sb24111201) detected by SVOM/ECLAIRs and SVOM/GRM (Yang et al., GCN 38173; Liu et al., GCN 38180). The observations were started at 11:46:00 (UT) on 2024-11-12 (~0.8 hr after SVOM trigger) and multiple images were obtained in uvgriz bands (as mentioned in Du et al., GCN 38176).

Based on the precise localization of the optical afterglow counterpart of GRB 241112B provided by SVOM/VT (Qiu et al., GCN 38179) and other facilities (Turpin et al., GCN 38181; Osborne et al., GCN 38182), we revisited the Mephisto data and identified the optical counterpart at this reported location. Initially, we missed this source while manually searching within the SVOM/ECLAIRs localization region due to its proximity to a faint, extended source in the DESI DR10 image and the lack of template subtraction, which led to confusion in identifying new transient sources.

Preliminary photometry of the optical afterglow of GRB 241112B (without template subtraction) in the griz bands and 3-sigma upper limits in the u and v bands are as follows:

Obs-start time band​ Exp(s) Mag (AB) 2024/11/12 11:46:00 i ​50*1 18.71 +/- 0.10​​ 2024/11/12 11:46:00 g 50*1 19.49​+/- 0.31 ​ 2024/11/12 11:56​:31 r 79*1 19.36 +/- 0.25​ 2024/11/12 11:56​:31 z 79*1 18.90​+/- 0.26​ 2024/11/12 11:46​:00 u 180*3 >21.21 2024/11/12 11:56​:31 v 180*3 >21.50 -------------------------------------------------------------------------- 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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