TITLE: GCN CIRCULAR NUMBER: 39136 SUBJECT: GRB 250129A: COLIBRÍ/DDRAGO Afterglow Detection DATE: 25/02/03 20:10:36 GMT FROM: Alan Watson at UNAM alan@astro.unam.mx

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

and the DDRAGO engineering team:

Luis Carlos Álvarez (UNAM), Fernando Angeles, Salvador Cuevas (UNAM), François Dolon (OHP), Alejandro Farah (UNAM), Johan Floriot (LAM), Jorge Fuentes-Fernández (UNAM), Arthur Langios (IRAP), Rosalía Langarica (UNAM), Simona Lombarda (LAM), Jaime Ruíz Díaz-Soto (UNAM), Samuel Ronayette (CEA), Silvio Tinoco (UNAM), and Hervé Valentín (IRAP)

report:

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

We observed with the DDRAGO wide-field science camera (Langarica et al., 2024, Proc SPIE 13096, 130963D) in a filter that closely approximates Pan-STARRS r, from 2025-02-03 07:45 to 08:00 UTC, at a midpoint of 5.20 days after the event, and obtained 600 seconds of exposure at a median airmass of 2.25 and good weather conditions. The data were reduced using custom software and then analysed and calibrated against the PS1 catalog using STDPipe (Karpov 2021).

We clearly detect the afterglow with

r = 22.10 +/- 0.15

This magnitude is not corrected for Galactic extinction. Our measurement is consistent with the magnitude reported by Bochenek & Perley (GCN Circ. 39131) at 4.93 days after the event.

Further observations are planned.

The DDRAGO camera is still being commissioned, and these are its first science observations. We warmly thank the COLIBRÍ engineering team and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.

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