AstroNote 2025-296

AT2025wny was first detected by the Zwicky Transient Facility (ZTF) on UT 2025-08-27 and given the identifier ZTF25abnjznp.  It was first reported to the TNS by the Gravitational-wave Optical Transient Observer (GOTO) on UT 2025-09-01 as GOTO25gtq.  The transient location is offset by approximately 1.5” from a candidate galaxy-scale strong lensing system (Cañameras et al. 2020, A&A, 644, 163). The deflecting galaxy has a spectroscopic redshift obtained by the Dark Energy Spectroscopic Instrument (DESI), placing it at z=0.3754.  Legacy Survey imaging (Dey et al. 2019, AJ, 157, 168) of the field shows a second galaxy nearby, likely at the same redshift, and deeper CFHT (Gwyn, S. D. J., 2008, PASP, 120, 212; Gwyn, S. D. J., 2012, AJ, 143, 38) imaging reveals four images of a blue background galaxy at unknown redshift in a cross-like pattern surrounding the candidate lens.

We observed the location of the transient on UT 2025-10-03 (MJD 60951.21) with the IO:O imager on the Liverpool Telescope (LT) for 2x100s in g-band, 2x90s in r-band and 2x120s in i-band, with ~1.2” seeing.  Image subtraction of the stacked frames against Pan-STARRS (Chambers et al. 2016, arXiv:1612.05560) reference imaging revealed clear detections of three separate point sources in all filters: the brightest source is located 1.5” to the northeast of the candidate lensing galaxy, and two fainter sources are visible approximately 2.1” south east and 1.6” south of the galaxy’s centre, respectively.  We again observed the transient with IO:O on UT 2025-10-04 (MJD 60952.22) for 2x150s in g, r and i with ~1.0” seeing. This again revealed detections of three separate sources in all three filters following image subtraction.  The three images detected are likely to be multiple gravitationally-lensed images of the same transient inside the background galaxy seen in the archival images.

From our imaging on the night of 2025-10-03, we measure the brightest image of the transient to have magnitudes of g = 20.42 ± 0.04, r = 19.60 ± 0.03, i = 19.54 ± 0.03.

The redshift and type of AT2025wny are currently unknown. We strongly encourage additional photometric and spectroscopic follow-up of this event. We are pursuing observations with HST and JWST (GO 17611; GO 5564,  PI: Goobar).

Based on observations made with the Liverpool Telescope operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.

Based on observations obtained with the Samuel Oschin Telescope 48-inch 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.

Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/IRFU, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at Terapix available at the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.