TITLE: GCN CIRCULAR NUMBER: 40210 SUBJECT: GRB 250419A: Mephisto Multi-band Optical Detection DATE: 25/04/22 17:40:55 GMT FROM: Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn> Edoardo Lagioia, Chenxu Liu, Yuan Fang, Guowang Du, Donglin Gao, Zhang Zepeng, Jinghua Zhang, Brajesh Kumar, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team: We conducted simultaneous multi-band photometric observations of the gamma-ray burst GRB 250419A (SVOM/ECLAIRs trigger sb25041901 at 2025-04-19T02:29:32 UTC; Wang et al. GCN 40168) with the Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University, located at the Lijiang Observatory. The observations extend over two nights from 19 to 20 April 2025. An optical counterpart is detected in the u-, g-, r-, i-band stacked images but not in the v-, z-band stack. The preliminary photometry and the 3σ upper limits are listed below. UT start |T-T0 (hr)| Band | Exp | Mag/LimMag (AB) -------------------|---------|------|--------|-------------------------- 2025/4/19 21:13:29 | 18.73 | u | 2x300s | > 20.63 2025/4/19 21:25:42 | 18.94 | v | 2x300s | > 20.91 2025/4/19 21:13:29 | 18.73 | g | 2x300s | 19.84 +/- 0.19 2025/4/19 21:25:42 | 18.94 | r | 2x300s | 19.92 +/- 0.10 2025/4/19 21:13:29 | 18.73 | i | 2x300s | 19.58 +/- 0.12 2025/4/19 21:25:42 | 18.94 | z | 2x300s | > 20.27 * Note: The above two sets of 300s exposures are taken at altitude ~ 23 deg. UT start |T-T0 (hr)| Band | Exp | Mag/LimMag (AB) -------------------|---------|------|--------|-------------------------- 2025/4/20 17:10:04 | 38.68 | u | 2x300s | 21.47 +/- 0.39 2025/4/20 17:10:06 | 38.68 | g | 2x300s | 21.00 +/- 0.17 2025/4/20 17:10:04 | 38.68 | i | 2x300s | 20.42 +/- 0.18 -------------------|---------|------|--------|-------------------------- 2025/4/20 19:50:05 | 41.34 | v | 1x300s | > 22.18 2025/4/20 19:50:04 | 41.34 | r | 1x300s | 21.15 +/- 0.29 2025/4/20 19:50:07 | 41.34 | z | 1x300s | > 21.30 Follow-up observations have already confirmed the presence of an optical counterpart ((López et al., GCN 40169; Xin et al., GCN 40170; Zheng et al., GCN 40171; Kumar et al., GCN 40172; Thakur et al., GCN 40174; Lipunov et al., GCN 40179; Odeh et al., GCN 40180; Perley & Bochenek, GCN 40181; Pankov et al., GCN 40182; Pérez-Fournon et al., GCN 40183; Kuin, GCN 40185; Wu et al., GCN 40186; Xie et al., GCN 40187; Jiang et al., GCN 40188; Ghosh et al., GCN 40189; Bochenek & Perley, GCN 40190; Pankov et al., GCN 40202; Gianluca Masi, GCN 40206; Giovanni Calapai & Massa S. Giorgio, GCN 40209). The ESO/X-Shooter spectroscopic observations identify a redshift of z = 0.845 (Thakur et al., GCN 40174). ---------------------------------------------------------- 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. ---------------------------------------------------------- View this GCN Circular online at https://gcn.nasa.gov/circulars/40210. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...