TITLE: GCN CIRCULAR NUMBER: 41328 SUBJECT: GRB 250812A: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 25/08/12 14:45:26 GMT FROM: Ny Avo Rakotondrainibe at LAM nyavo.rakotobe@gmail.com

Ny Avo Rakotondrainibe (LAM), Diego López-Cámara (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 Sánchez Álvarez (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), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), and L. P. Xin (NAOC):

We imaged the field of the SVOM GRB 250812A (Xin et al., GCN Circ. 41322) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-08-12 11:47 to 11:53 UTC (from 8.86 to 8.96 hours after the trigger) and obtained 300 seconds of exposure in the i-band filter. Due to the position of the field, the observations were conducted during astronomical twilight and at an airmass of about 3.4.

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 SkyMapper DR4 catalog, is in the AB system, and is not corrected for Galactic extinction.

We detected the optical counterpart reported by He et al. (GCN Circ 41324) and Xin et al. (GCN Circ 41326) at a preliminary magnitude of:

i = 18.89 +/- 0.05

Further observations are planned.

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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