GCN Circular 44384
Geoffrey Mo (Caltech/Carnegie), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Viraj Karambelkar (Columbia), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of GRB 260421B (Fermi GBM Team, GCN 44360; Preis et al., GCN 44361; Zhao et al., GCN 44362; Mukherjee et al., GCN 44382) 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 2026-04-21T04:36:00 UTC in the J band (~23 min after the GRB trigger) under poor conditions, consisting of 10 x 120 s 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).
We do not detect a source at the optical counterpart location (He et al., GCN 44363; Álvarez et al., GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Lipunov et al., GCN 44367; Schneider et al., GCN 44368; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383). We obtain the following 5-sigma upper limit: J = 16.4 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.