TITLE: GCN CIRCULAR NUMBER: 42073 SUBJECT: GRB 251003A: COLIBRÍ optical detection of the fading afterglow DATE: 25/10/03 03:55:12 GMT FROM: enriquemm@astro.unam.mx
Enrique Moreno Méndez (UNAM), Benjamin Schneider (LAM), Camila Angulo (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), 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), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM):
We imaged the field of the Swift GRB 251003A (Beardmore et al., GCN Circ. 42070) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-03 02:19 to 02:22 UTC (from 0.3 to 0.7 hours after the trigger) and obtained 3 minutes of simultaneous exposure in the r and z filters.
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 PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
We detected the optical counterpart reported by Aceituno et al., GCN Circ. 42072, at a position of RA, Dec = 18.52939, 56.49110 (J2000), with preliminary magnitudes of:
r = 20.34 +/- 0.11 z = 18.56 +/- 0.05
The object fades in subsequent observations, confirming that it is the optical afterglow.
The very red color suggests that this might be either a moderate redshift GRB with a drop-out in r or a very heavily reddened GRB.
Further observations in g/r/i/z/y 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.
View this GCN Circular online at https://gcn.nasa.gov/circulars/42073. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...