TITLE:   GCN CIRCULAR
NUMBER:  42420
SUBJECT: EP251023a: OHP/T193 optical observations
DATE:    25/10/24 13:25:17 GMT
FROM:    Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe@gmail.com>

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

We carried out observations of EP251023a (Mohan et al., GCN 42411; Selezneva et al., GCN 42406; Lipunov et al., GCN 42405; Mandarakas et al., GCN 42400; Liu et al., GCN 42391; Li et al., GCN 42389; Wu et al., GCN 42388; Wortley et al., GCN 42387) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 4 exposures of 15 min in the r-band at a midtime of 2025 10 24 03:17:52UT corresponding to T-T0 = 24.78 hours.

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

r’ = 21.55+/-0.10 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.