TITLE: GCN CIRCULAR NUMBER: 39412 SUBJECT: GRB 250221A: 1.6m Mephisto optical detection DATE: 25/02/21 18:35:37 GMT FROM: Brajesh Kumar at SWIFAR, YNU brajesh@ynu.edu.cn
Helong Guo, Guowang Du, Xinlei Chen, Xiaotong Chen, Yiheng Xie, Brajesh Kumar, Yuan Fang, Xingzhu Zou, Yu Pan (all SWIFAR, YNU), Xuhui Han, Pinpin Zhang, Liping Xin, Chao Wu (all NAOC), Yuanpei Yang, Jinghua Zhang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of Mephisto Team:
Simultaneous multi-band photometric observations of the Swift GRB 250221A (Caputo et al., GCN 39396; Beardmore et al., GCN 39404) was performed with 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. The observations were started from 11:55:52 2025-02-21 UT (~8.35 hr after the trigger) and several frames with different exposure time were obtained in uvgr bands. The afterglow candidate (Watson et al., GCN 39397; Melandri et al., GCN 39406; Shilling et al., GCN 39409) is clearly detected in the stacked images of g and r band but not in u and v bands. The preliminary magnitudes and 3-sigma upper limits are below:
Start_Time(UT) Filter Exp(sec) Mag/LimMag(AB) 2025-02-21T11:55:53 u 120*4, 300*3 >22.20 2025-02-21T12:05:54 v 120*3, 300*3 >22.61 2025-02-21T11:55:52 g 120*4, 300*3 21.57 +/- 0.26 2025-02-21T12:05:54 r 120*3, 300*3 20.87 +/- 0.13
---------------------------------------------------------------------------- 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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