TITLE: GCN CIRCULAR NUMBER: 37300 SUBJECT: GRB 240825A : MISTRAL/T193 OHP optical follow-up of the afterglow DATE: 24/08/26 15:52:26 GMT FROM: Emeric Le Floc'h at CEA-Saclay emeric.lefloch@cea.fr

E. Le Floc’h (CEA Paris-Saclay, DAp/AIM), C. Adami (LAM), B. Schneider (MIT), A. Saccardi (GEPI, Obs. De Paris), S. Basa (OSU Pytheas, LAM), M. Dennefeld (IAP), F. Schüssler (CEA Paris-Saclay, DPhP), report on behalf of the MISTRAL GRB collaboration :

We observed the field of GRB 240825A (Gupta et al., GCN 37274; Fermi GBM team, GCN 37273) with the MISTRAL instrument mounted on the 193cm telescope at Observatoire de Haute-Provence (France). Three exposures of 5min were obtained in the SDSS r’ band, for a total exposure time of 15min and an observation mid-time of 2024 August 25, ~22:40 UT (~6.85 hr after the GRB trigger).

The optical afterglow is clearly detected, at a position consistent with the counterpart reported earlier by other telescopes (e.g., Jiang et al., GCN 37275; Dutton et al., GCN 37276; Odeh et al., GCN 37277, Li et al., GCN 37280, Izzo et al., GCN 37287; Lipunov et al. GCN.37289; Leonini et al. GCN 37291; Wu et al., GCN 37292; Martin-Carrillo et al., GCN 37293; Brivio et al., GCN 37295; Odeh, GCN 37299).

We obtain a magnitude of r’ = 20.63 +/- 0.03 mag (AB), calibrated against nearby stars from the Pan-STARRS catalog, and where the photometric uncertainty does not include any systematics. The photometry is not corrected for Galactic extinction.

We are grateful to Xavier Delfosse (IPAG) and we also thank the support from the Observatoire de Haute-Provence, in particular Jean-Pierre Troncin and Jérome Schmitt.

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