TITLE: GCN CIRCULAR NUMBER: 44610 SUBJECT: GRB 260511B: Mephisto multi-band optical followup DATE: 26/05/14 15:51:44 GMT FROM: Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn> Chenxu Liu, Chenxi Shang, Guowang Du, Brajesh Kumar, Xian Liu, Haipeng Lei, Weikang Lin, Yu Pan, Xingzhu Zou, Xinlei Chen, Yuan Fang, Jinghua Zhang, Xufeng Zhu, Tao Wang, Xinzhong Er, Yuanpei Yang (SWIFAR, YNU), Chao Wu (NAOC), Xiangkun Liu, Xiaowei Liu (SWIFAR, YNU) report on behalf of the Mephisto Team: We performed simultaneous multi-band (uvgriz) photometric observations of GRB 260511B (Godet et al., GCN 44543) at 2026-05-12T16:17:44 (~1.2d after the the SVOM/ECLAIRs trigger) using the 1.6-m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory. The optical counterpart was clearly detected in our stacked images and preliminary photometry is listed below (without Galactic extinction correction): ------------------------------------------------------- Start Time (UT) | Band | Exp(s) | Mag (AB) ------------------------------------------------------- 2026-05-12T16:17:44 | u | 300*2 | 22.45 (+/-0.20) 2026-05-12T16:29:15 | v | 300*2 | 21.77 (+/-0.19) 2026-05-12T16:17:47 | g | 300*2 | 21.19 (+/-0.08) 2026-05-12T16:29:17 | r | 300*2 | 21.03 (+/-0.07) 2026-05-12T16:17:46 | i | 300*2 | 20.70 (+/-0.11) 2026-05-12T16:29:17 | z | 300*2 | 20.40 (+/-0.16) These results are consistent with previous optical observations reported by Wu et al. (GCN 44547), Sasada et al. (GCN 44549), Jiang et al. (GCN 44551), Wu et al. (GCN 44552), Vijaykumar et al. (GCN 44561), de Ugarte Postigo et al. (GCN 44565), Lee et al. (GCN 44567), Li et al. (GCN 44568), Wang et al. (GCN 44571), Belkin et al. (GCN 44572), Adami et al. (GCN 44573), Saccardi et al. (GCN 44574), Rajabov et al. (GCN 44576), Perez-Fournon et al. (GCN 44577), Novotny et al. (GCN 44580), Moskvitin et al. (GCN 44582), Calapai et al. (GCN 44592), and Moskvitin et al. (GCN 44600). ---------------------------------------------------------------------------------------------------- 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. The facility is operated by the South-Western Institute for Astronomy Research (SWIFAR), Yunnan University. It provides real-time, high-quality colors of stellar objects. The Mephisto mosaic cameras were installed in October 2025. The first light was achieved in all three channels on 10 October 2025 and presently, these are under the commissioning phase. All the data have been reduced by the Mephisto data processing pipeline. We note that the current data-processing pipeline is still at a preliminary stage, with flux calibration precision of about 2% in the u and v bands, about 1% in the g and r bands, and better than 1% in the i and z bands. View this GCN Circular online at https://gcn.nasa.gov/circulars/44610. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...