TITLE: GCN CIRCULAR NUMBER: 43991 SUBJECT: GRB 260310A/AT 2026fgk: DDOTI Optical Detection DATE: 26/03/13 14:38:23 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra@astro.unam.mx> Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report: We observe the field of the Fermi/GBM GRB 260310A (Fermi GBM Team, GCN 43951) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the nights of 2026-03-10 and 2026-03-11 UTC. DDOTI observed from 05:11 UTC to 9:34 on 2026-03-10 (from T+0.2 hours to T+4.6 hours after the trigger) with a total exposure time of 2.1 hours and from 03:32 UTC to 12:20 on 2026-03-11 (from T+22.6 hours to T+31.4 hours after the trigger), with a total exposure time of 3.1 hours. On both nights we detect the source AT2026fgk (discovered by GOTO; O’Neill et al. 2026, TNS Discovery Report 294132), proposed as the optical counterpart (Konno et al.; GCN Circ. 43974), with preliminary AB magnitudes of: w ≈ 16.6 (in our first epoch); w = 17.52 +/- 0.01 (in our second epoch). The photometry of the second night is consistent with a temporal decay of alpha≈-0.6 between the magnitudes reported by the LAST collaboration (Konno et al.; GCN Circ. 43974) and COLIBRÍ (Moreno Méndez et al.; GCN Circ. 43980). Due to technical problems during the first night, the photometry from that epoch should be considered tentative. These values are not corrected for the Galactic extinction. We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir. View this GCN Circular online at https://gcn.nasa.gov/circulars/43991. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...