TITLE: GCN CIRCULAR NUMBER: 42421 SUBJECT: GRB 251023B: OHP/T193 optical observations DATE: 25/10/24 13:26:31 GMT FROM: Ny Avo Rakotondrainibe at LAM nyavo.rakotobe@gmail.com

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

We carried out observations of GRB 251023B (Cheung et al., GCN 42407; Mandarakas et al., GCN 42404; Nakahira et al., GCN 42402; Evans et al., GCN 42401; Kang et al., GCN 42392) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager. We obtained 6 exposures of 15 min in the r-band at a midtime of 2025 10 24 01:51:23 UT, corresponding to T-T0 = 17.04 hours.

We do not significantly detect the optical source identified by Mandarakas et al. (GCN 42404) at the 3-sigma upper limit of:

r’ < 21.78 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.

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