TITLE:   GCN CIRCULAR
NUMBER:  40837
SUBJECT: GRB 250625A: COLIBRÍ optical observations
DATE:    25/06/26 09:35:11 GMT
FROM:    F. Fortin at IRAP <ffortin.sci.edu@gmail.com>


Francis Fortin (IRAP), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (AUS), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):

We imaged the field of the Fermi/Swift co-detection of GRB 250625A (Fermi GBM Team, GCN Circ. 40824, Ferro et al., GCN Circ. 40825) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-06-26T03:42:43 to 05:49:43 UTC (from 11.41 to 13.53 hours after the trigger) and obtained 32 minutes of exposure in each of the g, r and i 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 Moskvitin et al. (GCN 40828), Xin et al. (GCN 40830), and Zhu et al. (GCN 40834), at a magnitude of:

g = 24.22 +/- 0.34
r = 23.43 +/- 0.21
i = 24.01 +/- 0.58

Further observations 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.