TITLE: GCN CIRCULAR NUMBER: 44043 SUBJECT: GRB 260310A / AT2026fgk: LCO optical observations DATE: 26/03/18 06:59:09 GMT FROM: Sergey Belkin at Monash University <sergey.belkin@monash.edu> S. Belkin and M. Shrestha (Monash U) report: We observed the optical counterpart AT2026fgk of GRB 260310A, detected by Fermi/GBM (Fermi GBM Team, GCN 43951) and AstroSat/CZTI (Salunke et al., GCN 43958), with the Las Cumbres Observatory 1m telescope at McDonald Observatory equipped with the Sinistro instrument. Observations began on 2026-03-14 at 06:12:09 UT, approximately 4.05 days after the trigger. We obtained 4 x 60 s exposures in the r band and 6 x 60 s exposures in the u band. In the stacked images we clearly detect the reported optical counterpart AT2026fgk (e.g. Lipunov et al., GCN 43954; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Mendez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo, GCN 44003; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022), discovered by GOTO (O’Neill et al., TNS Discovery Report 294132). The preliminary photometry from the stacked images is: Date (UT) T-T0 (days) Filter Exp. time (s) Magnitude 3-sigma UL ======================================================================================== 2026-03-14 06:14:13 4.0535 r 60 x 4 18.58 +/- 0.02 21.79 2026-03-14 06:21:49 4.0588 u 60 x 6 19.47 +/- 0.04 21.12 The r-band photometry is calibrated against Pan-STARRS field stars. The u-band photometry is calibrated against Gaia DR3 synthetic SDSS u-band stars. The quoted values are not corrected for Galactic extinction. Further follow-up is encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/44043. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...