TITLE: GCN CIRCULAR NUMBER: 44003 SUBJECT: GRB 260310A / AT2026fgk: OASDG optical observations DATE: 26/03/14 09:12:46 GMT FROM: luca.izzo@inaf.it L. Izzo (INAF-OACn and DARK/NBI) reports: We observed the likely counterpart of GRB 261013A (Fermi GBM team, GCN #43951; Hamburg et al., GCN #43975) using the 0.5-m T1 telescope of the Osservatorio Astronomico S. Di Giacomo, located in Agerola, Italy. Our observations started on 2026 March 14 at 00:04 UT, 3.797 days after the GRB trigger. We acquired a series of 8 × 300 s images in the Rc and Ic filters. The optical counterpart, also reported as AT2026fgk (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; 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) is clearly detected in the individual frames. In the Rc-band stacked image, we measure a preliminary magnitude of Rc = 18.21 +/- 0.07 mag (AB), calibrated against nearby stars from the Pan-STARRS catalog and appropriately transformed to Rc magnitudes. The measured magnitude may still be affected by contamination from the bright host galaxy, but further analysis is ongoing. View this GCN Circular online at https://gcn.nasa.gov/circulars/44003. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...