TITLE: GCN CIRCULAR NUMBER: 39288 SUBJECT: IceCube-250207A: One candidate from the Zwicky Transient Facility DATE: 25/02/11 16:35:05 GMT FROM: Jannis Necker at DESY jannis.necker@desy.de

Jannis Necker, Akshay Eranhalodi (DESY), Robert Stein (JSI), Sven Weimann (Ruhr University Bochum), 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-250207A (Sommani et. al, GCN 39203) 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 2025-02-08 06:52 UTC, approximately 28.7 hours after event time. We covered 98.9% (9.8 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 | +--------------------------------------------------------------------------------+ | ZTF25aafgnar | ------- | 131.6422069 | +19.3306551 | r | 20.68 | 0.16 | +--------------------------------------------------------------------------------+

ZTF25aafgnar was first detected on 2025-01-19. The source is somewhat red, with a g-r colour of 0.3. Due to the offset of 0.92 arcsec from the nucleus of its host SDSS J084634.07+191950.9, it appears most likely to be a supernova.

Forced photometry reveals ZTF pre-detections beginning on 2025-01-19, demonstrating that the supernova predates the detection of IC250207A. The source reached a peak magnitude of m=20.2 in g-band on 2025-01-23, and has since faded by 0.5 mag. Photometric redshift estimates for the host galaxy from Legacy Survey suggest a distance of z = 0.26 +/- 0.06, with a 95% lower limit of z>0.165, implying a peak absolute magnitude of at least M<-19.4.

The colour and implied absolute magnitude are consistent with a supernova origin for this source.

The timing of the neutrino detection excludes any choked-jet neutrino production models, but CSM-interaction neutrino production models would still be viable.

Spectroscopic observations are planned to determine the nature of this transient.

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 ).

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