TITLE: GCN CIRCULAR NUMBER: 44365 SUBJECT: GRB 260421B: LCO optical afterglow confirmation DATE: 26/04/21 05:32:17 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> D. Turpin (CEA/Irfu), D. B. Malesani (DAWN/NBI and Radboud), A. Saccardi (CEA/Irfu), C. Wu (NAOC), J. T. Palmerio (CEA/Irfu), L. P. Xin (NAOC), B. Cordier (CEA/Irfu), W.-L Zhang (PMO), Z.-Y Liu (GZNU), report on behalf of the SVOM mission team: We observed the field of GRB 260421B detected by SVOM (Zhao et al., GCN 44362) and Fermi/GBM (The Fermi GBM team, GCN 44360) with the LCO 1m telescope at Cerro Tololo Observatory equipped with the Sinistro instrument. Our observation started at 2026-04-21T04:29:54.940 about 16.5 min after the SVOM/ECLAIRs trigger time. We obtained 3x200 s exposures in the SDSS r and PanSTARRS-z filters. An uncatalogued bright optical candidate is well detected in our single images at RA, dec (J2000) = 190.00203, -17.74017 (err = 0.5 arcsec). This position is consistent with the optical counterpart reported by the TRT telescope (He et al, GCN 44363) and the SVOM/COLIBRI FM-GFT (Álvarez et al. GCN 43646). We measure the following magnitude calibrated against the PanSTARRS DR1 catalog, and not corrected for Galactic extinction: r = 18.47 +/- 0.01 (AB) (Tmid - T0 = 18.2 min) z = 18.92 +/- 0.09 (AB) (Tmid - T0 = 30.7 min) This project is funded by the SVOM collaboration. Further observations are scheduled. View this GCN Circular online at https://gcn.nasa.gov/circulars/44365. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...