TITLE: GCN CIRCULAR NUMBER: 40421 SUBJECT: IceCube-250506A: No Transient Candidates from the Zwicky Transient Facility DATE: 25/05/10 16:32:23 GMT FROM: Robert David Stein at JSI <rdstein@umd.edu> Akshay Eranhalodi (DESY), Robert Stein (JSI), Jannis Necker (DESY), Anna Franckowiak (Ruhr University Bochum) and Jesper Sollerman (Stockholm) 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-250506A (Zegarelli et. al, GCN 40369) 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 2025-05-08 04:24 UTC, approximately 38.2 hours after event time. Observations were delayed due to poor weather. We covered 78.5% (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 candidate, lying within the 90.0% localization of the skymap. +--------------------------------------------------------------------------------+ | ZTF Name | IAU Name | RA (deg) | DEC (deg) | Filter | Mag | MagErr | +--------------------------------------------------------------------------------+ | ZTF23aabbszx | AT2023fyh | 116.1542766 | +35.3663416 | r | 19.29 | 0.17 | +--------------------------------------------------------------------------------+ ZTF23aabbszx is a nuclear source, and was first detected on 2023-01-25. It has a spectrum in DESI, with a redshift of z=0.3860 and clear AGN signatures. It similarly has AGN-like WISE colours of W1-W2=0.76. Based on this, we conclude that ZTF23aabbszx is an AGN. The source has been undergoing a long period of outburst, beginnning more than 800 days ago, with at least two distinct flares. The most recent flare began 300 days ago, and the source has been fading for the past 50 days. There is no particular activity in the past 6 months which suggest a connection to the high-energy neutrino. 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; 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 ). View this GCN Circular online at https://gcn.nasa.gov/circulars/40421. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...