TITLE: GCN CIRCULAR NUMBER: 44119 SUBJECT: GRB 260310A / AT2026fgk: R- and I-band observations with the PKU 60 cm optical telescope DATE: 26/03/26 13:15:35 GMT FROM: Y.C. Kang at Peking University <yckang@stu.pku.edu.cn> Yacheng Kang, Qiang Wang, Ruize Shi, Zexuan Wu, Zhuokai Liu, Yiming Dong, Ziming Wang, Chenxi Bao, Fangzhou Ren, Lijing Shao, Zhuo Li, Subo Dong (PKU) report on behalf of the PKU HiTF (High-energy Transients Follow-up) group: The field of GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al., GCN 43958) was observed with the PKU 60 cm optical telescope at Xinglong Observatory, NAOC. Observations began on 2026 Mar 18, corresponding to ~8.6 days after the GRB trigger, and were conducted over a total of three epochs. In the stacked R- and I-band images, we clearly detect the optical counterpart AT2026fgk (e.g., Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Malesani et al., GCN 43990; Becerra et al., GCN 43991; Li et al., GCN 43993; Stein et al., GCN 43996; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo et al., GCN 44003; Brivio et al., GCN 44004; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin et al., GCN 44043; Li et al., GCN 44044; Gupta et al., GCN 44051; Passaleva et al., GCN 44058; Becerra et al., GCN 44059; Volnova et al., GCN 44060; Busmann et al., GCN 44061; Zheng et al., GCN 44065; Brosio et al., GCN 44067; Pankov et al. GCN 44073; Perley et al. GCN 44096; Passaleva et al. GCN 44104; Serrau et al. GCN 44112; O'Neill et al. 2026, TNS Discovery Report 294132). Preliminary photometry yields R-band magnitudes ranging from ~18.7 to ~18.9 mag and I ~ 18.5 mag, calibrated against nearby Pan-STARRS field stars and reported in the Vega system, without correction for Galactic extinction. The observation log is summarized below (T0 = 2026-03-10T04:57:10.81; GRB 260310A): | Date | Start_UT | T_mid − T0 (days) | Filter | Exposure (s) | Magnitude (Vega) | | :--------: | :---------: | :---------------: | :----: | :----------: | :--------------: | | 2026-03-18 | 20:23:26.87 | 8.66 | R | 4 * 600 | 18.66 +/- 0.03 | | 2026-03-19 | 16:22:31.97 | 9.53 | I | 15 * 600 | 18.45 +/- 0.03 | | 2026-03-19 | 18:57:02.55 | 9.63 | R | 14 * 600 | 18.90 +/- 0.03 | Further analysis and follow-up observations are ongoing. The 60 cm telescope is operated by the Department of Astronomy, School of Physics, Peking University. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients. View this GCN Circular online at https://gcn.nasa.gov/circulars/44119. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...