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.