TITLE: GCN CIRCULAR NUMBER: 40838 SUBJECT: GRB 250625A: OHP/T193 optical detection DATE: 25/06/26 11:02:44 GMT FROM: Christophe Adami at LAM christophe.adami@lam.fr

C. Adami (LAM/Pytheas/AMU), S. Basa (LAM/OHP/Pytheas/AMU), B. Schneider (LAM), E. Le Floc'h (CEA/Irfu), A. Ugarte Postigo (LAM), M. Dennefeld (IAP/CNRS/Sorbonne U.), report on behalf of a larger collaboration:

We observed the field of the GRB 250625A (Fermi GBM Team, GCN 40824; Ferro et al., GCN 40825; Beardmore et al., GCN 40827; Dichiara et al., GCN 40835; Di Lalla et al., GCN 40836) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 8 exposures (for a total of 81min) in the r-band starting at 20:34:39 UT on 2025-06-25 (4.50 hours after the trigger). We also obtained 2 exposures (for a total of 30min) in the i-band starting at 22:28:55 UT on 2025-06-25 (6.40 hours after the trigger). In the stacked images, we detected the optical counterpart reported/followed by Moskvitin et al., GCN 40828, Xin et al., GCN 40830; Malesani et al., GCN 40831; Lipunov et al., GCN 40826/40832; Zhu et al., GCN 40834, Fortin et al., GCN 40837; in the r-band image. The preliminary magnitude derived for that source is:

r = 22.95 +/- 0.23 mag (AB)

We do not detect the optical counterpart in the i-band image with a 3-sigma upper limit of: i < 22.2

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 R.P. Nelson for the MISTRAL observations.

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