TITLE: GCN CIRCULAR NUMBER: 45125 SUBJECT: IceCube-260708A: One candidate from the Zwicky Transient Facility DATE: 26/07/09 16:21:49 GMT FROM: Robert David Stein at JSI <rdstein@umd.edu> Robert Stein (JSI), Tomas Ahumada (Noirlab), Mansi Kasliwal (Caltech), Jesper Sollerman (Stockholm), Cristobal Zilleruelo Cañas (DESY), Jannis Necker (Leiden University), Akshay Eranhalodi (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-260708A (Zegarelli et. al, GCN 45120) 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 2026-07-09 08:12 UTC, approximately 14.9 hours after event time. We covered 23.8% (0.2 sq deg) of the reported localization region at least twice. 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. +--------------------------------------------------------------------------------+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | +--------------------------------------------------------------------------------+ | ZTF26abfurzw | AT2026kuv | 292.8653948 | -15.1344488 | r | 20.11 | 0.07 | +--------------------------------------------------------------------------------+ ZTF26abfurzw/AT 2026kuv was discovered by GOTO on 2026-04-20, and reported to TNS. ATLAS forced photometry reveals a lightcurve rising since discovery, later peaking at 17.4 mag at the beginning of May. The source has since slowly faded. The source was detected in our ZTF ToO observations, and the photometry confirms that the source is now very red (g-r = 1) and faded substantially from peak. There were no previous ZTF survey detections, because the source lies in a chip gap for the survey footprint. The photometric properties (rise, fade, and late-time colour) of AT 2026kuv appear consistent with a supernova. Neutrino emission in a post-peak supernova would be plausible if there were CSM interaction in the source. We encourage spectroscopic observations of this source to confirm its nature, and to search for evidence of CSM interaction. Further monitoring of this field will continue as part of the ZTF neutrino follow-up program (Stein et al. 2023), and we are planning additional follow-up of AT 2026kuv. 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 ). View this GCN Circular online at https://gcn.nasa.gov/circulars/45125. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...