TITLE: GCN CIRCULAR NUMBER: 39104 SUBJECT: GRB 250129A: COLIBRÍ Optical Observations DATE: 25/01/31 07:13:32 GMT FROM: Alan Watson at UNAM alan@astro.unam.mx

Alan M. Watson (UNAM), Camila Angulo (UNAM), D. Akl (AUS), S. Antier (OCA), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Rosa Becerra (Università degli Studi di Roma Tor Vergata), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), J.-G. Ducoin (CPPM), Francesco Magnani (CPPM) and Margarita Pereyra (UNAM) report:

We imaged the field of the Swift GRB 250129A (Beardmore et al., GCN Circ. 39066) during the commissioning of the COLIBRÍ (SVOM/F-GFT) telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.

We observed with the OGSE engineering test camera in a red filter that approximates SDSS r from 2025-01-30 07:01 to 12:54 UTC. In the subset of our data from 2025-01-30 12:01 to 12:54, from 31.27 to 32.15 hours after the event, with a total exposure of 2220 seconds, we clearly detect the afterglow with

r = 19.38 +/- 0.04

This magnitude is not corrected for Galactic extinction. The data were reduced using custom software and then analysed and calibrated against the PS1 catalog using the STDWeb service.

Further observations are planned.

We warmly thank the COLIBRÍ engineering team and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir. We warmly thank the GRANDMA IJCLAB team and S. Karpov for the access of the STDWeb service for STDPipe.

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