GCN Circular 38707

Sven Weimann (Ruhr University Bochum), Jannis Necker (DESY), Robert Stein (JSI), and Anna Franckowiak (Ruhr University Bochum) 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-241224A (Zegarelli et. al, GCN 38664
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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 observations in the g- and r-band beginning at 2024-12-24 10:31 UTC, approximately 3.4 hours after event time. We covered 75.3% (1.1 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 find one candidate lying 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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| ZTF18aaxzhgj |  -------  | 185.0307759 | +02.4088207 | g      | 18.31 | 0.06   | 
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ZTF18aaxzhgj was first detected on 2018-06-05. The optical detection is located 0.17 arcsec from the nucleus of known AGN 2MASX J12200736+0224320. This AGN has a spectroscopic redshift of z=0.159 from SDSS, and a QSO broadline classification.

The source has been slowly brightening over the past five years, reaching a difference imaging magnitude of g ~ 18. In reference science images from PS1 (Chambers et al. 2016), the source was detected at a slightly fainter level of g = 19, demonstrating that the source has brightened in recent years.

However, there are no indications of significant flaring on timescales of either weeks or months that coincide with the detection of IC241224A. We therefore find no strong evidence from our data to suggest that this AGN is associated with the neutrino. 

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 Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Oskar Klein Center at Stockholm University, the University of Maryland, University of California, Berkeley , the University of Wisconsin at Milwaukee, University of Warwick, Ruhr University Bochum, Cornell University, Northwestern University and Drexel University. Operations are conducted by COO, IPAC, and UW. 

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 ).