TITLE: GCN CIRCULAR NUMBER: 45509 SUBJECT: GRB 260903A: OHP/T193 optical afterglow detection DATE: 26/09/04 01:52:51 GMT FROM: Christophe Adami at LAM <christophe.adami@lam.fr> D. Russeil (LAM/Pytheas/AMU), C. Adami (LAM/Pytheas/AMU), E. Le Floc'h (CEA/Irfu), A. Saccardi (CEA/Irfu), F. Schüssler (CEA/DPhP), S. Basa (LAM/OHP/Pytheas/AMU) report on behalf of a larger collaboration: We observed the field of the SVOM/ECLAIRs GRB 260903A (Brunet et al., GCN 45499) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 19 exposures of 300s (for a total of 95 min) in the r-band starting at 20260903_223831 UT (observations mid-time at T-To=11h). Target was close to the moon. In the stacked images, we detect the optical counterpart reported and followed by Li et al. (GCN 45504), Saccardi et al. (GCN 45505), and Malesani et al. (GCN 45508). The preliminary magnitude derived for that source is: r’ = 23.0 +/- 0.2 mag (AB) The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction. We used the STDWeb/STDPipe tools (Karpov 2025). We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular S. Favard. View this GCN Circular online at https://gcn.nasa.gov/circulars/45509. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...