GCN 44003: GRB 260310A / AT2026fgk: OASDG optical observations

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.