TITLE:   GCN CIRCULAR
NUMBER:  43996
SUBJECT: GRB 260310A/AT 2026fgk: WINTER NIR Detections
DATE:    26/03/13 21:05:01 GMT
FROM:    Robert David Stein at JSI <rdstein@umd.edu>

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

We observed AT 2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65; O’Neill et al. 2026, TNS Discovery Report 294132), identified as the likely counterpart to GRB 260310A (Fermi GBM Team, GCN 43951). We used 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 2026-03-12T09:57:09.115 UTC (~53 hours after the GRB trigger), and continued over several expochs in both J-band and Hs-band. 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 clearly detect a source in all of our stacked images, with times given relative to burst time (2026-03-10T04:57:11):

| t - t0 (hr) | filter | mag(AB) | magerr | limiting mag |
|-------------|--------|---------|--------|--------------|
|    53.00    |   J    |  17.16  |  0.09  |     19.28    |
|    53.58    |   J    |  17.28  |  0.09  |     19.18    |
|    54.13    |   Hs   |  17.10  |  0.21  |     17.91    |
|    71.87    |   J    |  17.34  |  0.12  |     19.03    |
|    72.17    |   Hs   |  16.97  |  0.20  |     17.77    |

We do not perform image subtraction due to a lack of suitable reference images, so there might be some contamination from the host galaxy. However, given the large offset and relatively faint host, we do not expect this to substantially alter our measurements.

We do not measure significant evolution over our ~20 hour baseline, after accounting for the uncertainty in each measurement. A slow fading would be consistent with these observations, and with expectations for a power-law decay of an afterglow at this relatively late (>2 days) phase. 

Our observations are consistent with values reported by Li et al (GCN 43993), who quote a value of J=16.7 +/- 0.1 mag in the Vega system (~17.6 mag AB) at a later phase of +81 hours.

Further observations of this transient are planned.

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.