HOLISMOKES XIX: SN 2025wny at $z=2$, the first strongly lensed superluminous supernova

TL;DR

SN 2025wny is the first galaxy-scale strongly lensed superluminous SN (SLSN) with time delays expected on days to weeks, enabling a new avenue for independent $H_0$ measurements via time-delay cosmography. Through coordinated imaging and spectroscopy, the authors establish a two-galaxy deflector (G1+G2) at spectroscopic redshifts $z_ ext{d}=0.3754$ and $z_ ext{p}=0.375$, and a SN redshift of $z_ ext{SN}=2.008 \\pm 0.001$, classifying it as a UV-bright SLSN-I with a high ejecta temperature of $\\gtrsim 17000$ K. The UV-dominated spectrum, four-image lens configuration, and measured delays provide a compelling laboratory for lens modeling and cosmography, with ongoing and planned follow-up (including HST/JWST) to enable precise time-delay measurements and mass modeling. The work also discusses selection effects that make lensed SLSNe more detectable than naive rates would suggest, highlighting the potential yield of lensed SLSNe in future surveys like LSST for refined cosmological constraints.

Abstract

We present imaging and spectroscopic observations of supernova SN 2025wny, associated with the lens candidate PS1 J0716+3821. Photometric monitoring from the Lulin and Maidanak observatories confirms multiple point-like images, consistent with SN 2025wny being strongly lensed by two foreground galaxies. Optical spectroscopy of the brightest image with the Nordic Optical Telescope and the University of Hawaii 88-inch Telescope allows us to determine the redshift to be z_s = 2.008 +- 0.001, based on narrow absorption lines originating in the interstellar medium of the supernova host galaxy. At this redshift, the spectra of SN 2025wny are consistent with those of superluminous supernovae of Type I. We find a high ejecta temperature and depressed spectral lines compared to other similar objects. We also measure, for the first time, the redshift of the fainter of the two lens galaxies (the "perturber") to be z_p = 0.375 +- 0.001, fully consistent with the DESI spectroscopic redshift of the main deflector at z_d = 0.3754. SN 2025wny thus represents the first confirmed galaxy-scale strongly lensed supernova with time delays likely in the range of days to weeks, as judged from the image separations. This makes SN 2025wny suitable for cosmography, offering a promising new system for independent measurements of the Hubble constant. Following a tradition in the field of strongly-lensed SNe, we give SN 2025wny the nickname SN Winny.

Figures (6)

• Figure 1: $r$-band image of SN 2025wny, obtained at the Lulin Observatory with LOT on September 29, 2025 (left), Pan-STARRS1 reference image (middle), and difference image after subtraction (right). Very bright stars have been masked. The images cover an area of roughly $9$ arcmin$^2$. The SN images A to D are labelled on a zoomed-in view of the difference image.
• Figure 2: $VRI$-band colour composite of SN 2025wny obtained at the Maidanak Observatory. The four visible images of the SN are labelled by A, B, C and D (in order of decreasing brightness), and the two foreground deflector galaxies, by G1 and G2. Top: observed image. Bottom left: image of the deflectors after subtracting SN and host light using Moffat profiles (with joint shape parameters). Bottom right: image of SN after subtracting G1 and G2's light using a single Sérsic profile each.
• Figure 3: Archival CFHT image from 2005 2012Gwyn, showing the two deflector galaxies G1 and G2 and four strongly lensed images of the SN host galaxy. A colour image generated from COLIBRI-telescope $riz$-band data is included to facilitate the comparison between the positions and brightnesses of the different SN and host-galaxy images.
• Figure 4: Spectra of SN 2025wny, obtained with NOT + ALFOSC and UH88 + SNIFS. A spectrum of the SLSN-I SNLS-06D4eu from howell2013 is included for comparison, plotted once with and once without offset relative to the SN 2025wny spectrum of October 11.16. SNLS-06D4eu provides the best match with SN 2025wny. Line identifications for SNLS-06D4eu have been adopted from howell2013 and mazzali2016, but we note that in SN 2025wny, the absorptions marked by the blue-shaded bands are weaker and more strongly blueshifted. The inserts at the bottom zoom in onto narrow absorption lines from the ISM in the host of SN 2025wny. They have been used to determine the redshift of the SN to be $z_{\rm SN} = 2.008 \pm 0.001$.
• Figure 5: Spectrum of the second deflector galaxy G2 ('perturber'), rebinned to a $10\,\text{\AA}$ bin size. An SDSS DR5 template spectrum of an early-type galaxy, shifted to a redshift $z_\mathrm{p} = 0.375 \pm 0.001$, is superimposed. Ca ii H&K, the Balmer break, the G band, Mg i, and Na i D are clearly detected.