TITLE:   GCN CIRCULAR
NUMBER:  42460
SUBJECT: GRB 251025B: OHP/T193 optical detection
DATE:    25/10/26 21:20:15 GMT
FROM:    Christophe Adami at LAM <christophe.adami@lam.fr>

N.A. Rakotondrainibe  (LAM), C. Adami (LAM/Pytheas/AMU),  E. Le Floc'h (CEA/Irfu), S. basa (UAR Pytheas) report on behalf of the MISTRAL GRB collaboration

We carried out observations of the GRB 251025B (​​Hussein et al., GCN 42437) optical and NIR afterglow (Wu et al., GCN 42438;  Gress et al., GCN 42439; Wu et al., GCN 42440;  Beardmore et al., GCN 42445; Mohan et al., GCN 42447; Hernández Funget al., GCN 42450; Pereyra et al., GCN 42452; Li et al., GCN 42453; Mo et al., GCN 42459) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 1 exposure of 3 min and 5 exposures of 12 min in the r-band at a midtime of 2025-10-26 20:11:53 UT corresponding to T-T0 = 29.8 hours.

The afterglow is well detected in r’ and we measured the following preliminary magnitude:

r’ = 20.93+/-0.07 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 Jean Balcaen and the SOPHIE observer Guillaume Hebrard.