TITLE: GCN CIRCULAR NUMBER: 41832 SUBJECT: GRB 250912A: SVOM/COLIBRÍ (FM-GFT) optical observations DATE: 25/09/14 15:36:39 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Sarah Antier (OCA), Damien Dornic (CPPM), Benjamin Schneider (LAM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), N. A. Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the SVOM/Fermi GRB 250912A (Ducoin et al., GCN Circ. 41820, and Fermi GBM Team, GCN Circ. 41819) using the DDRAGO two-channel wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2025-09-13 05:51 to 08:33 UTC (from 8.75 to 11.45 hours after the ECLAIRs trigger) and obtained 112 minutes of simultaneous exposure in the r and z filters and then again from 2025-09-14 05:52 to 08:10 UTC (from 32.78 to 35.07 hours after the ECLAIRs trigger) and obtained 96 minutes of simultaneous exposure in the r and z filters. 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 do not detect the J-band source reported by Garnichey et al. (GCN Circ. 41929) and Ricci et al. (GCN Circ. 41830) to 3-sigma limits at our first epoch of: r > 24.8 z > 23.7 and at our second epoch of: r > 24.7 z > 23.5 This confirms the absence of a similarly bright optical counterpart to the J-band source and confirms its extremely red colors. We have subtracted our two epochs and in the difference image find no clear evidence for an optical transient in the MXT error region (Ducoin et al., GCN Circ. 41820) at our first epoch to 5-sigma limits of: r > 23.9 z > 22.8 Our result is in agreement with earlier observations reported by Lipunov et al. (GCN Circ 41822), Wu et al. (GCN Circ. 41824), and Li et al. (GCN Circ. 41825). Evans et al. (GCN Circ. 41831) report the discovery of an uncataloged X-ray source outside of the MXT error region. This source is close to a galaxy (object 121410150406866805 in the Pan-STARRS DR2 catalog) with r ≈ 19.6 and z ≈ 18.9. At this position in our difference image, we do not see an obvious optical transient, but our ability to quantify this is limited by significant subtraction artifacts. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/41832. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...