TITLE:   GCN CIRCULAR
NUMBER:  42339
SUBJECT: GRB 251017A: OHP/T193 i-band observations
DATE:    25/10/17 22:08:45 GMT
FROM:    Christophe Adami at LAM <christophe.adami@lam.fr>

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

We carried out observations of the GRB 251017A (Gupta et al., GCN 42322; Perez-Garcia et al., GCN 42323; Antier et al., GCN 42325; Goad et al., GCN 42326; Sasada et al., GCN 42329; Lipunov et al., GCN 42330; Fu et al., GCN 42331; Woolf et al., GCN 42335; Mo et al., GCN 42336) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 3 exposures of 3min and 2 exposures of 10min in the i-band at a mid time of 2025-10-17 19:52:19UT, corresponding to T-T0 = 11.19 hr. 

The afterglow is detected and we measured the following preliminary magnitude:

i’ = 21.39+/-0.32 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 Yoann Degot-longhi and the SOPHIE observers H. Bouy