TITLE: GCN CIRCULAR NUMBER: 40478 SUBJECT: GRB 250515A:COLIBRÍ optical observations of the GOTO candidate DATE: 25/05/17 06:16:42 GMT FROM: Alan Watson at UNAM <alan@astro.unam.mx> Kin Ocelotl López (UNAM), Alan M. Watson (UNAM), Rosa L. Becerra (U Roma), Benjamin Schneider (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Noémie Globus (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Antonio de Ugarte Postigo (LAM): We imaged the field of the GOTO candidate afterglow (Gompertz et al., GCN Circ. 40464) of the Fermi GRB 250515A (Fermi GBM team., GCN Circ. 40462) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-05-17 03:23 to 05:27 UTC (median epoch 1.30 days after the trigger) and obtained 32 minutes of exposure in the i filter and 3 minutes in each of the g and r 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 detected the optical candidate at preliminary magnitudes of: g = 20.31 +/- 0.03 r = 20.24 +/- 0.04 i = 20.45 +/- 0.03 This source was also detected by SVOM/VT (Xin et al., GCN Circ. 40474) and Mephisto (Zhang et al., GCN Circ. 40476). Our magnitudes are similar in g, fainter in r, and brighter in i than those reported at 0.71 days by Zhang et al. (GCN Circ. 40746), and these changes are not obviously compatible with the temporal evolution of a GRB afterglow after about 1 day. 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/40478. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...