TITLE: GCN CIRCULAR NUMBER: 43979 SUBJECT: GRB 260310A / AT2026fgk: CAHA 1.23m telescope optical observations DATE: 26/03/13 02:28:03 GMT FROM: Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie> A. Martin-Carrillo, J. Roca Garcia-Valino, E. Aasen, L. Brown, D. Colfer, M. Collins, J. Costello, C. Harris, R. Odunuga, H. O'Leary Jeffers, C. O'Malley, J. Parimkayala, R. Roddy, D. Scott, A. Valdivia y Alvarado Carvajal (UCD), J. F. Agui Fernandez (CAHA), A. Fernandez-Martin (CAHA) report on behalf of a larger collaboration: We observed the optical counterpart candidate (Konno et al., GCN 43974; Lipunov et al. GCN 43978, discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132) of GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al. 2026, GCN 43958) using the CAHA 1.23m telescope equipped with the DLR-MkIII CCD camera. A series of 3 x 150s exposures were taken in the BVR bands starting at 2026-03-12T22:57:09 (~2.75 days after the Fermi trigger). The candidate is well detected in the individual images with a preliminary AB magnitude of R = 18.2 +/- 0.1. Our photometry was calibrated using nearby stars from the Pan-STARRS catalogue and is not corrected for Galactic extinction. It should be noted that it is likely that this value has some contribution from the nearby likely host galaxy. 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/43979. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...