GCN 43497: IceCube-260115A: No Candidate Transients from the Zwicky Transient Facility

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-260115A (Zegarelli et. al, GCN 43421) 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 2026-01-16 05:53 UTC, approximately 12.3 hours after event time. We covered 99.2% (8.2 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 one transient candidate by our pipeline 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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| ZTF26aabnqai | AT2026azb | 145.5760425 | +13.0239392 | r      | 21.08 | 0.15   |
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ZTF26aabnqai was first detected on 2026-01-16 at an apparent magnitude of about 21 mag in r-band. The g-r color is around 0.5 mag. Forced photometry revealed previous detections up to a week earlier with a fading rate of about 0.5 mag per day. The host galaxy has a photometric redshift of 0.23 +/- 0.08 in Legacy Survey. During our latest observatiosn the transient was no longer detectable, consistent with continued fading. This makes the transients too faint for our spectroscopic programs. However, the color, fade time and inferred absolute magnitude are all broadly consistent with a Type Ia supernova. These are not expected to emit high-energy neutrinos and thus we consider ZTF26aabnqai unlikely to be related to IceCube-260115A.

We further identify following quasars without contemporaneous flaring activity. The hosts of ZTF22aaejlfn (SDSS J094112.55+131224.9), ZTF25aadwsdp (SDSS J093659.79+124616.6), ZTF25aadxqgk (SDSS J094449.16+120545.5), and ZTF25aarudkx (SDSS J094755.84+125346.4) are listed in MILLIQUAS (Flesch et al., 2023). ZTF24abwkvpl was first detected on 2024-11-05. Its host, SDSS J093633.62+131948.5, is part of a selection of quasars by SDSS photometry (Richards et al., 2009). The hosts of ZTF25aaedxfb, and ZTF25aadxmes show red, AGN-like WISE colors with W1-W2>0.8 (Stern et al., 2012). ZTF25abttvfc was consistently detected over the last 100 days with varying brightness. ZTF18acqfhyp, ZTF25aadnych, and ZTF25aadoovh were sporadically detected before the neutrino arrival and are likely variable sources at the sensitivity limit.

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).
Alert filtering is performed with the nuztf (Stein et al. 2021, https://github.com/desy-multimessenger/nuztf ).