TITLE: GCN CIRCULAR NUMBER: 41055 SUBJECT: IceCube-250708A: No Candidate Transients from the Zwicky Transient Facility DATE: 25/07/10 01:13:48 GMT FROM: Robert David Stein at JSI rdstein@umd.edu
Jannis Necker (DESY), Akshay Eranhalodi (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-250708A (Zegarelli et. al, GCN 41039) 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 2025-07-09 04:37 UTC, approximately 14.5 hours after event time. We covered 82.2% (0.7 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.
+--------------------------------------------------------------------------------+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | +--------------------------------------------------------------------------------+ | ZTF25abanmjp | ------- | 222.4281885 | +26.5431350 | r | 20.63 | 0.13 | +--------------------------------------------------------------------------------+
ZTF25abanmjp was first detected on 2025-06-10. It is nuclear, with a WISE-detected host (W1-W2=0.58). With these colours, this galaxy is a likely AGN. These detections are brighter than archival PS1 photometry of this galaxy (g=22.1, r=21.5), indicating an elevated flux level. However, forced potometry at this position reveals many previous detections at this position in ZTF data over the past six months, with no obvious flaring activity at the time of the neutrino. We conclude that ZTF25abanmjp likely arises from AGN variability. Given the lack of obvious flaring at the time of the neutrino, we have no reason to think that ZTF25abanmjp is associated with IC250708A based on our observations.
Observations of this field will continue 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, and OKC, University of Stockholm, Sweden. 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 ).
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