TITLE:   GCN CIRCULAR
NUMBER:  44958
SUBJECT: GRB 260610B / AT 2026owq: COLIBRÍ further observations and confirmation of the rebrigthening
DATE:    26/06/16 19:58:21 GMT
FROM:    Rosa Leticia Becerra Godínez at Instituto de Astronomía,  UNAM <rbecerra@astro.unam.mx>

Camila Angulo (UNAM), Marion Guelfand (CPPM), Rosa L. Becerra (UNAM), Nikos Mandarakas (LAM), Antonio de Ugarte Postigo (LAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Dalya Akl (NYUAD), Jean-Luc Atteia (IRAP),Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), and Alan M. Watson (UNAM) report:

We reimaged the field of AT 2026owq (O'Neill et al., GCN Circ. 44903, 44914; Watson et al., GCN Circ. 44905; Zhu et al., GCN Circ. 44909; Gillanders et al., GCN Circ. 44910; Akl et al., GCNs 44911, 44930; Moskvitin et al., GCNs 44918, 44929; Jackson-Horne et al., GCN Circ. 44919; Izzo et al., GCN Circ. 44920; Li et al., GCN Circ. 44921; Bochenek et al., GCNs 44927, 44932; Pankov et al., GCN Circ. 44946; Gillanders  et al., GCN Circ. 44995), which is the optical counterpart of GRB 260610B (Fermi GBM team, GCN Circ. 44901), using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. In our latest epoch, we observed from 2026-06-16 07:26 to 08:41 UTC (from 5.32 to 5.37 days after the GRB trigger) and obtained 6, 4, 5, and 14 minutes of exposure in the g, r, i, and z filters, respectively.

The data were reduced, coadded, calibrated, and analysed with the COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR2 catalog, is in the AB system, and is not corrected for Galactic extinction.

We detect the source AT2026owq with preliminary magnitudes of:

g = 21.52 +/- 0.13, 
r = 21.02 +/- 0.10,
i = 20.69 +/- 0.09, 
z = 20.43 +/- 0.10.

Compared with previous observations reported for this event, we also observe the rebrightening/flattening first reported by the Pan-STARRS team (Gillanders et al., GCN Circ. 44995), beginning approximately 80 hours after the trigger and visible in all of our filters.

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 México (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, México.