TITLE: GCN CIRCULAR NUMBER: 44172 SUBJECT: GRB 260310A / AT2026fgk: Luminance-band optical follow-up with a 28cm telescope in Daocheng DATE: 26/04/01 07:07:05 GMT FROM: Y.C. Kang at Peking University <yckang@stu.pku.edu.cn> Zhuokai Liu, Zexuan Wu, Chenxi Bao, Yacheng Kang (PKU) report on behalf of the Interestar Collaboration and the PKU HiTF (High-energy Transients Follow-up) group: We observed the field of GRB 260310A, detected by Fermi/GBM (GCN 43951; T0 = 2026-03-10T04:57:10.81) and AstroSat/CZTI (Salunke et al., GCN 43958), using a 28cm optical telescope located in Daocheng, Sichuan, China. Observations began on 2026 Mar 17, approximately ~ 7.4 days after the GRB trigger, and spanned six epochs in the Luminance filter. In the stacked Luminance-band images, we detect the optical counterpart at a position consistent with AT2026fgk, the reported GRB afterglow candidate at z = 0.153 (O'Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65; Konno et al., GCN 43974; Hinds et al., GCN 43977), later found to show Type Ic-BL SN features (de Ugarte Postigo et al., GCN 44124; Guelfand et al., GCN 44125; O'Connor et al., GCN 44137). Preliminary photometry was calibrated using nearby catalogued field stars and is reported as approximate AB-equivalent magnitudes, without correction for Galactic extinction. Our measurements show that the optical counterpart continues to fade, from ~ 19.0 mag to ~ 19.8 mag between 7.4 and 14.3 days after the GRB trigger. The observation log is summarized below: | Date | Start_UT | T_mid - T0 (days) | Filter | Exposure (s) | Magnitude (AB) | | :--------: | :------: | :---------------: | :-------: | :----------: | :------------: | | 2026-03-17 | 13:28:40 | 7.40 | Luminance | 7400 | 19.02 +/- 0.04 | | 2026-03-18 | 13:52:51 | 8.42 | Luminance | 7600 | 19.10 +/- 0.04 | | 2026-03-19 | 14:14:50 | 9.42 | Luminance | 6000 | 19.18 +/- 0.04 | | 2026-03-20 | 12:37:02 | 10.36 | Luminance | 7400 | 19.39 +/- 0.04 | | 2026-03-21 | 12:39:41 | 11.35 | Luminance | 5000 | 19.65 +/- 0.05 | | 2026-03-24 | 12:33:31 | 14.33 | Luminance | 2200 | 19.84 +/- 0.08 | Further analysis and follow-up observations are ongoing. The 28cm optical telescope is located in Daocheng, Sichuan, China, and is operated by the Interestar Collaboration. 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/44172. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...