TITLE: GCN CIRCULAR NUMBER: 40306 SUBJECT: EP250427a / GRB 250427A: OHP/T193 photometric observations: third visit DATE: 25/05/01 16:12:13 GMT FROM: Christophe Adami at LAM christophe.adami@lam.fr

C. Adami (LAM/Pytheas/AMU), S. Basa (Pytheas/OHP/LAM), N.A. Rakotondrainibe (LAM), S. Vergani (CNRS, Obs. de Paris, LUX), A. Saccardi (CEA/Irfu), B. Schneider (LAM), J. T. Palmerio (CEA/Irfu) report on behalf of the MISTRAL-GRB collaboration:

We observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) a third time using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting.

We obtained 1x72sec + 1x600sec + 1x900sec + 4x720sec in the r’ band starting at 01:34:13UT on 2025-05-01 (T0+93.9 h after the trigger). In the stacked image, we do not significantly detect the optical afterglow reported by Pérez-García et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273, Wang et al. GCN40274, Amram et al. GCN 40275, Siegel et al. GCN 40279, Amram et al. GCN 40288, Eappachen et al. GCN 40289, and Ghosh et al. GCN 40293 down to the following 5-sigma upper limit: r > 22.8. If we force the detection at the EP250427a afterglow position, we get a very marginal detection just below the detection limit with a preliminary magnitude of r=23.1+/-0.35

The photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.

We acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen, and the SOPHIE observer Owin Scutt.

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