TITLE: GCN CIRCULAR NUMBER: 42266 SUBJECT: GRB 251013C: OHP/T193 optical afterglow photometry and spectroscopy DATE: 25/10/14 19:29:02 GMT FROM: Christophe Adami at LAM christophe.adami@lam.fr

C. Adami (LAM/Pytheas/AMU), B. Schneider (LAM), E. Le Floc'h (CEA/Irfu), A. de Ugarte Postigo (LAM), S. Basa (Pytheas/OHP/LAM), F. Schüssler (CEA/Irfu), N.A. Rakotondrainibe (LAM), report on behalf of a larger collaboration:

We observed the field of GRB 251013C detected by FERMI (Fermi GBM Team, GCN Circ. 42221) and SVOM/ECLAIRs (Rakotondrainibe et al., GCN Circ. 42222) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained one exposure of 3min in the r-band starting at 19:28:38 UT on 2025-10-13. The optical counterpart previously reported by different groups is well detected. Consistent with previously reported photometric measurements, the preliminary magnitude derived for that source is:

r’ = 15.92 +/- 0.04 mag (AB)

We also obtained a 10min spectrum roughly covering the 4800-8200A range, starting at 19:19:40 UT 2025-10-13. We detect a strong continuum but no prominent lines in our range, consistently with the redshift measured by Martin-Carrillo (GCN 42227).

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 Jean Balcaen and the SOPHIE observers J.B. Salomon and J.L. Halbwachs

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