TITLE: GCN CIRCULAR NUMBER: 41356 SUBJECT: GRB 250813A: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 25/08/14 05:51:16 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS adeugartepostigo@gmail.com

Ramandeep Gill (UNAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Noémie Globus (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), W.J. Xie (NAOC), and B.T. Wang (YNAO, CAS):

We imaged the field of the SVOM GRB 250813A (W.J. Xie et al., GCN Circ. 41352) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-08-14 04:05:30 to 04:44:19 UTC (mean epoch 5.5618 hours after the trigger) and obtained 32x60 s of exposure in the i filter.

The data were reduced and coadded with the COLIBRÍ pipeline and analyzed 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.

We detected the optical counterpart reported by Jelinek et al., GCN Circs. 41353 and 41355, at a preliminary magnitude of:

i = 21.38 +/- 0.09

Further observations are ongoing.

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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