GCN Circular 39638

Robert Stein (JSI), Jannis Necker, Akshay Eranhalodi (DESY), and Anna Franckowiak (Ruhr University Bochum), Tomas Ahumada (Caltech), Jesper Sollerman (Stockholm) report:

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations:

As part of the ZTF neutrino follow up program (Stein et al. 2023), we observed the localization region of the neutrino event IceCube-250309A (Zegarelli et. al, GCN 39631
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 ) with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started deep observations in the g- and r-band beginning at 2025-03-09 10:16 UTC while the same field was also observed by the routine ZTF survey at 08:18 UTC, approximately 0.7 hours after event time. We covered 92.9% (0.3 sq deg) of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 21.0 mag.

The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019). We are left with one candidate within the 90.0% localization of the skymap.

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| ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr |
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| ZTF25aaiurnn | –––- | 211.3537073 | -10.4938701 | g | 20.82 | 0.18 |
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ZTF25aaiurnn was first detected on 2025-03-02 in the r-band, roughly one week before the neutrino arrival. It has a match in the MILLIQUAS catalog at a distance of 0.18 arcsec as a 95% probable QSO. The match in the AllWISE source catalog (WISEA J140524.90-102938.0, 0.22 arcsec) shows a red color of W1-W2=1), consistent with an AGN. Although there is only one detection per band, there is no clear evidence for coincident flaring activity.

We will continue to observe this field as part of our standard ToO cadence for high-energy neutrinos (Stein et al. 2023).

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.

GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.
Alert distribution service provided by DIRAC@UW (Patterson et al. 2019).
Alert database searches are done by AMPEL (Nordin et al. 2019).
Alert filtering is performed with the nuztf (Stein et al. 2021, https://github.com/desy-multimessenger/nuztf ).