TITLE: GCN CIRCULAR NUMBER: 41972 SUBJECT: GRB 250924A: J-band detection with WINTER DATE: 25/09/24 19:38:12 GMT FROM: Geoffrey Mo at Caltech / Carnegie Observatories gmo@mit.edu

Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (Caltech), Viraj Karambelkar (Columbia/CCA), Benjamin Schneider (LAM), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:

We observed the field of GRB 250924A (Siegel et al., GCN 41959) 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 2025-09-24T08:26:28 UTC in the J band (~8 minutes after the GRB trigger), consisting of 15 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 detect a clear source at the optical counterpart location (Siegel et al., GCN 41959; Becerra et al., GCN 41960; Lipunov et al., GCN 41962; Schneider et al., GCN 41963; Freeberg et al., GCN 41968; Pankov et al., GCN 41969; Zheng et al., GCN 41970; Saikia et al., GCN 41971), with magnitude J ~ 18.6 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.

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