GCN Circular 35584

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

We observed the localization region of the neutrino event IceCube-240105A (Sommani et. al, GCN 35498
Not found
 
 
) with the Palomar 1-m telescope, equipped with the 1 square degree WINTER camera (Lourie et al 2020). We started observations in the J-band beginning at 2024-01-06T05:22:36 UTC, approximately 16.9 hours after event time. As a result of the evolving neutrino localisation (GCN 35485
Not found
 
 
, GCN 35498
Not found
 
 
) and inclement weather, our observations took place over four nights (Jan 5th, 6th, 12th, 16th), with some overlap in coverage between visits. In total, we covered ~90% of the reported localization region at least once, and ~35% at least twice. This estimate does not account for chip gaps. Exposures had a median depth of 18.5 mag AB.

The images were processed through the WINTER data reduction pipeline implemented using mirar (Stein, Karambelkar et al in prep.,  https://github.com/winter-telescope/mirar), with image subtraction performed relative to reference images built from the UKIRT Hemisphere Survey (Dye et al. 2017). We extract sources with positive difference flux.

Three methods were used to identify time-varying sources in our data.
First, we apply loose image quality cuts to all 10,789 WINTER candidate sources, without requiring multiple detections, and this yields 1,607 candidates. Second, we separately cross-match all 10,789 WINTER detections to optical detections from the Zwicky Transient Facility (ZTF, Bellm et al 2019) with a radius of 3 arcseconds, and select sources that are detected by both WINTER and ZTF. We recover 12 candidates with a ZTF crossmatch. Third, we select sources detected at least twice by WINTER, and recover 56 candidates. There is some overlap in candidates between these three selection methods.

After visual scanning of all these candidates to reject subtraction artefacts, and removing probable stars with measured Gaia DR3 parallax and sources with matches in the Minor Planet Center, we recover one candidate counterpart in our observations. WNTR24aabfj, spatially coincident with blazar PKS 0446+11, passes all three candidate selection methods. 

The blazar has already been flagged as a possible counterpart to IC240105A (GCN #35499, GCN #35517, GCN #35548, ATEL #16397, ATEL #16398, ATEL #16399, ATEL #16402, ATEL #16407, ATEL #16409), undergoing a substantial flare across the EM spectrum. Given that this is a variable source, we perform forced photometry directly on our WINTER science images to measure the flux of this source in our data. We detect the following AB magnitudes for this blazar in our data:

—----------------------------------------------------
| Date (UTC)          | Mag (AB) | Mag Err | Filter |
—----------------------------------------------------
| 2024-01-06T05:22:36 |  15.10   |  0.04   |   J    |
| 2024-01-07T02:43:37 |  15.56   |  0.07   |   J    |
| 2024-01-17T02:38:36 |  15.83   |  0.06   |   J    |
—----------------------------------------------------

For comparison, the reference flux of this source in 2MASS J-band is 17.9 mag (AB), indicating a clear increase in NIR flux relative to this historical baseline. Even over the course of our WINTER observations, the source has faded by >0.7 magnitudes from peak across our three epochs. 

Further observations of the field will continue.

WINTER (Widefield 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

Alert querying for coincident ZTF detections is done with AMPEL (Nordin et al. 2019) and nuztf (Stein et al. 2021, https://github.com/desy-multimessenger/nuztf ).