GCN 45198: IceCube-260722A: One candidate from the Zwicky Transient Facility (ZTF)

Jannis Necker (Leiden University), Akshay Eranhalodi (DESY), Robert Stein (JSI), Cristobal Zilleruelo Cañas (DESY), 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-260722A (Zilleruelo Cañas et. al, GCN 45194) 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 2026-07-23 04:54 UTC, approximately 21.9 hours after event time. We covered 88.8% (2.3 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 the following high-significance transient candidate by our pipeline, 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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| ZTF26abjbxfs |  AT 2026vts  | 191.3022538 | +30.5970446 | r      | 19.78 | 0.07   |  
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ZTF26abjbxfs / AT 2026vts was first detected on 2026-07-23, and is very blue (g-r = -0.2). It appears to lie on the edge of galaxy SDSS J124512.84+303550.1, with a spectroscopic redshift of z=0.0978. Assuming that this is the redshift of the transient, it has a current absolute magnitude of M=-18.8. ATLAS forced photometry reveals two marginal (3 sigma) detections in the ATLAS o-band at m= 19.6 +/- 0.2 mag, 3-4 days before the ZTF observations. This suggests that the transient is still blue more than four days after first detection. 

We encourage spectroscopic follow-up to determine its nature and assess potential high-energy neutrino emission scenarios. 

Further monitoring of this field will continue as part of the ZTF neutrino follow-up program (Stein et al. 2023)

ZTF and GROWTH are worldwide collaborations comprising Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, 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; OKC, Sweden; DZA, Germany.

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 ).