TITLE: GCN CIRCULAR NUMBER: 42521 SUBJECT: IceCube-251025A: Two Transient Candidates from the Zwicky Transient Facility DATE: 25/10/29 23:11:01 GMT FROM: Robert David Stein at JSI <rdstein@umd.edu> Akshay Eranhalodi (DESY), Robert Stein (JSI), Jannis Necker (Leiden University) 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-251025A (Zegarelli et. al, GCN 42436) 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 2025-10-26 11:02 UTC, approximately 21.2 hours after event time. We covered 98.9% (2.1 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 are left with the following high-significance transient candidates by our pipeline, all lying within the 90.0% localization of the skymap. ±–––––––––––––––––––––––––––––––––––––––-+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | ±–––––––––––––––––––––––––––––––––––––––-+ | ZTF25acammho | AT 2025abwy | 144.8380039 | +15.6961873 | g | 21.13 | 0.21 | | ZTF25acamsai | AT 2025abtz | 146.0869065 | +14.6023944 | r | 20.75 | 0.12 | ±–––––––––––––––––––––––––––––––––––––––-+ Amongst our candidates, we find two unclassified transients of potential interest. ZTF25acammho/AT 2025abwy was inititally detected on 2025-10-03 in r-band, and was then redetected in our ToO observations three weeks later. The source appears to be hostless, with a current magnitude of 21.13. ZTF25acamsai/AT 2025abtz was first detected on 2025-10-22, and appears coincident with a host galaxy of photometric redshift z=0.2. This would imply a magnitude of M=-19.5. The source appears to be blue. We encourage spectrscopic observations to confirm the nature of these two transients. In addition, we also recover one known supernova, ZTF25abvepyj/SN2025zno. The source was first detected by ZTF on 2025-10-02, and is classified as Type Ia Supernova at a redshift of 0.064. Type Ia supernovae are not predicted to be significant neutrino emitters, so we conclude that this source is unrelated to the neutrino. We further find one likely AGN (ZTF19aaixfxw) with W1-W2=0.8. This source does not display any evidence of flaring coincident with the neutrino, and we see no reason to associate it with IC251025A. The neutrino localisation was also observed by DDOTI (Becerra et al., GCN 42457) who report no uncatalogued source with w > 20. These observations are consistent with our observations reported here. ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA; WIS, Israel; OKC, Sweden; JSI/UMd, USA; DESY, Germany; TANGO, Taiwan; UW Milwaukee, USA; LANL, USA; TCD, Ireland; IN2P3, France. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/42521. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...