TITLE: GCN CIRCULAR NUMBER: 45501 SUBJECT: GRB 260903A: LCO optical counterpart detection DATE: 26/09/03 15:11:23 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> Y.X. Guan(GXU), A. Saccardi (CEA/Irfu), S.J.Yin (GXU), C. Wu, L.P. Xin (NAOC), Yunyu Shi (GXU), D. Turpin (CEA/Irfu), R.-C. Chen (NJU), T. Xiao (GXU) report on behalf of the SVOM mission team: We observed the field of the SVOM/ECLAIRs GRB 260903A (Brunet et al., GCN 45499) with the LCO 1m telescope at Siding Spring Observatory equipped with the Sinistro instrument. Our observation started on 2026-09-03T13:06:41.486 UT (about 29.36 min after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters. An uncatalogued optical source compared to DESI DR10 catalogue is clearly detected in our images within the ECLAIRs localization error circle (Brunet et al., GCN 45499). Its position is R.A., Dec. 25.77241, 11.43678 degrees: R.A. (J2000) = 01h43m05.37 Dec. (J2000) = +11d26m12.4 with an uncertainty of ~0.5 arcsec. We measure the following magnitudes calibrated against the DESI DR10 catalog, and not corrected for Galactic extinction: r = 17.91 +/- 0.02 AB (mid-time 38.66 min after the trigger); z = 17.69 +/- 0.13 AB (mid-time 31.01 min after the trigger). This project is funded by the SVOM collaboration. View this GCN Circular online at https://gcn.nasa.gov/circulars/45501. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...