TITLE: GCN CIRCULAR NUMBER: 43103 SUBJECT: GRB 251214B: COLIBRÍ observations of the afterglow DATE: 25/12/15 04:27:56 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Alan M. Watson (UNAM), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), Camila Angulo (UNAM), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), 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), Nikos Mandarakas (LAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the Fermi/Swift GRB 251214B (Fermi GBM Team, GCN Circ. 43088; Eyles-Ferris et al., GCN Circ. 43089) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-15 01:33 to 03:22 UTC (from 16.52 to 18.34 hours after the Swift/BAT trigger) and obtained 28, 28, 28, and 84 minutes of simultaneous exposure in the g, r, i, and z filters, respectively. The data were reduced, coadded, and analyzed with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In our stacked images, at the position of the afterglow (Eyles-Ferris et al., GCN Circ. 43089; Kang et al., GCN Circ. 43093; Gress et al., GCN Circ. 43095; Fu et al., GCN Circ. 43097; and Gupta et al., GCN Circ 43100) we detect the a source with preliminary magnitudes of: g = 22.97 +/- 0.21 r = 22.05 +/- 0.12 i = 21.52 +/- 0.12 z = 20.97 +/- 0.08 We note the presence of a bright galaxy coincident with the afterglow in Legacy Survey observations (Dey et al. 2019) with g = 23.70 +/- 0.11, r = 22.37 +/- 0.06, i = 21.62 +/- 0.06, z = 21.12 +/- 0.04, and a photometric redshift of z = 0.70 +/- 0.07. We suggest this might be the host galaxy. Its low redshift is consistent with the detection of the afterglow in g by Kang et al. (GCN Circ. 43093) and in our observations. Our observations include light from both the afterglow and this galaxy, and the afterglow is only clearly detected above the galaxy in g. The fading in g between the observations of Kang et al. (GCN Circ. 43093) and our observations implies a temporal power-law index of 1.08 or steeper. 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/43103. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...