GCN 43587: GRB 260131A/B (AT2026bwg): COLIBRÍ optical observations and confirmation of fading

We observed the optical afterglow candidate AT2026bwg discovered by Gompertz et al. (GCN 43586),  of GRB 260131A (Fermi/GBM Team, GCN 43579), also associated with GRB 260131B (Sugai et al., GCN 43580; Roberts et al., GCN 43585), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-01 02:06 to 03:05 UTC (from 19.45 to 20.46 hours after the trigger) and obtained 23 minutes of simultaneous exposure in the r and z filters.

The data were reduced, coadded, calibrated, and analyzed with the COLIBRÍ 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.

The optical counterpart is clearly detected with preliminary magnitudes of:

r = 17.96 +/- 0.01,
z = 17.61 +/- 0.01.

Since the start of our observations, the source has faded with an apparent average temporal power-law index of -2.01 +/ 0.26 (in r) and -1.50 +/- 0.33 (in z).

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.