GCN Circular 36403

Robert Stein (Caltech), Geoffrey Mo (MIT), Danielle Frostig (MIT), Viraj Karambelkar (Caltech), Nathan Lourie (MIT), Tomas Ahumada (Caltech), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report, on behalf of the WINTER collaboration:

We continued observations of the localization region of the LVK trigger S240422ed (GCN 36236
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) with the 1.2 sq. degree WINTER camera on the Palomar 1-m telescope (Lourie et al. 2021). We obtained images in the near-IR J-band of the Bilby sky map (GCN 36240
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) on 5 nights between 2024-04-22 and 2024-04-30. Our observations covered the same fields that we previously reported (GCN 36248
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), covered ~14% of the localization probability, and achieved a 5-sigma depth ranging between m_J = 16 - 17 mag (AB) depending on the sensor temperature. The observations have been reported to TreasureMap (Wyatt et al. 2020).

The images were processed through the WINTER data reduction pipeline implemented using mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.10888436
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). Our science images were then subtracted against J-band reference images built from the UKIRT survey (Dye et al. 2017) or VISTA survey (Sutherland et al. 2015). We find 66351 candidates detected in at least one image. 

Of these, 160 candidates are detected at least twice by WINTER and pass quality cuts to reject bogus detections and stars. However, after visual inspection, we reject all candidates as either stellar, AGN variability or subtraction artefacts.

We also cross-matched all candidates within the alert stream of the Zwicky Transient Facility (GCN 36246
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). We found 48 candidates detected at least once in WINTER and at least once in ZTF. However, after visual inspection, we also reject all these candidates as either stellar, AGN or subtraction artefacts.

We therefore find no candidate transients in our WINTER observations.

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 Lucille Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.