GCN Circular 45406

We observed the field of GRB 260818A (Yin et al., GCN 45391

) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations began at 2026-08-18T07:40:58.776 UTC in the J band (6.61 hr after the SVOM/ECLAIRs trigger), consisting of 15*120 sec exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).

Of the five unique uncatalogued X-ray sources (EPF_J231953.3-320838 is repeated in the FXT circular) reported by FXT (Wang et al., GCN 45400

), four lie within our WINTER image, with EPF_J231912.9-320034 falling outside the field of view. Of these four sources, two are coincident with catalogued sources: EPF_J231936.3-320537 is classified as a star, while EPF_J231953.3-320838 is classified as a quasar in the Gaia catalog. We also note that EPF_J231953.3-320838 has a measured redshift of z=0.1898 in the GAMA survey (Yao et al. 2020; https://ui.adsabs.harvard.edu/abs/2020ApJ...903...91Y/abstract). We do not detect counterparts at the positions of the remaining two FXT sources (Ducoin et al., GCN ; Shi et al., GCN ). We obtain a 5-sigma upper limit of J=19.14 mag (AB).

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.