TITLE: GCN CIRCULAR NUMBER: 41302 SUBJECT: GRB 250808A: SVOM/COLIBRÍ (FM-GFT) optical upper limit DATE: 25/08/09 17:04:52 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS adeugartepostigo@gmail.com

Antonio de Ugarte Postigo (LAM), Noémie Globus (UNAM), Jean-Grégoire Ducoin (CPPM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), D. Adrien (CEA) and D. Turpin (CEA):

We imaged the field of the SVOM GRB 250808A (Adrien et al., GCN Circ. 41297) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-08-09 11:11 to 11:50 UTC (from 16.8 to 17.4 hours after the trigger) and obtained 32x60 s of exposure in the i filter. The observations were carried out mostly during the astronomical twilight and with an average airmass ~ 2.7.

The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

Within the full SVOM/ECLAIRs error circle we do not detect any new source down to a 5-sigma limiting magnitude of i > 21.4 mag. More specifically, within the error circle of the XRT afterglow (Evans et al. GCN Circ. 41300) we do not detect any significant optical counterpart down to the following 3-sigma limit:

i > 22.0 mag

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

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