TITLE: GCN CIRCULAR NUMBER: 44000 SUBJECT: GRB 260310A / AT2026fgk: further CAHA 1.23m telescope optical observations suggest shallow decay DATE: 26/03/14 04:43:17 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie> A. Martin-Carrillo, E. Aasen, L. Brown, D. Colfer, M. Collins, J. Costello, C. Harris, R. Odunuga, H. O'Leary Jeffers, C. O'Malley, J. Parimkayala, J. Roca Garcia-Valino, R. Roddy, D. Scott, A. Valdivia y Alvarado Carvajal (UCD), A. Fernandez-Martin (CAHA), J. F. Agui Fernandez (CAHA), report on behalf of a larger collaboration: We observed the position of AT2026fgk, the likely counterpart to GRB 260310A (Fermi GBM team, GCN 43951; Salunke et al., GCN 43958; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; 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; discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132) using the CAHA 1.23m telescope equipped with the DLR-MkIII CCD camera. A series of 3 x 150s exposures were taken in the BVRI bands starting at 2026-03-13T23:59:00 (~3.793 days after the Fermi trigger). The candidate is still well detected in the individual images with a preliminary AB magnitude of R = 18.3 +/- 0.1. Our photometry was calibrated using nearby stars from the Pan-STARRS catalogue and is neither corrected for Galactic extinction nor the host galaxy contribution. Comparing this new epoch with our previously reported R-band magnitude (Martin-Carrillo et al., GCN 43979) we infer a shallow decay with a power-law index of ~0.3, significantly shallower than the decay observed (~0.7) since the initial report by Konno et al., GCN 43974. These observations were made during the UCD Academy observing run at Calar Alto Observatory (Almeria, Spain) as part of the final year undergraduate projects of the University College Dublin BSc (Hons) in Physics with Astronomy and Space Science. View this GCN Circular online at https://gcn.nasa.gov/circulars/44000. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...