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Supernova 2025wny: High-angular resolution Keck/NIRC2 observations and preliminary lens modeling

Christopher J. Storfer, Kenneth C. Wong, Ana Acebron, Claudio Grillo, Willem B. Hoogendam, Xiaosheng Huang, David O. Jones, Eugene A. Magnier, Kaisey S. Mandel, Nicolas Ratier-Werbin, David Rubin, Benjamin J. Shappee, Oscar Soler-Perez

Abstract

Multiply imaged, gravitationally lensed supernovae are rare but powerful tools for providing independent measurements on cosmological parameters. Supernova (SN) 2025wny ("SN Winny") is the first gravitationally-lensed Type I superluminous supernova and the first lensed supernova in a galaxy-scale system that is suitable for time-delay cosmography studies. In this work, we present high-resolution $K_p$-band adaptive optics imaging of SN Winny obtained with the near-infrared camera (NIRC2) on the W. M. Keck II telescope. With exquisite image quality (FWHM$\approx0.^{\prime\prime}065$) we determine and make use of the precise astrometric positions of the five multiple images as constraints for our lens mass models. With lenstronomy and Glee, we parameterize the total mass of the system with a singular isothermal ellipsoid, a singular isothermal sphere, and external shear. The two independent models are in excellent agreement and reproduce the observed image positions with sub-milli-arcsecond residuals. The inferred projected total masses enclosed within the Einstein radii of the primary and secondary lens galaxies are M$_1$ = 4.44$^{+0.06}_{-0.05}\times10^{11} M_\odot$ and M$_2$ = 0.96$^{+0.02}_{-0.02}\times10^{11} M_\odot$, respectively. Likewise, the inferred effective velocity dispersion of the primary lens is $σ_{1} = $ 277.4$^{+0.9}_{-0.7}$ km/s, consistent with the independent spectroscopic measurement made by DESI of $σ_{\star,1} = $ 298$\,\pm\,37$ km/s. Our modeling results are also consistent with previous results for the same system with data from the Large Binocular Telescope (LBT), using the same lens modeling codes. We also corroborate their finding that the SN multiple image A has an anomalous excess of flux by a factor of ~2-3 beyond what our smooth mass models predict.

Supernova 2025wny: High-angular resolution Keck/NIRC2 observations and preliminary lens modeling

Abstract

Multiply imaged, gravitationally lensed supernovae are rare but powerful tools for providing independent measurements on cosmological parameters. Supernova (SN) 2025wny ("SN Winny") is the first gravitationally-lensed Type I superluminous supernova and the first lensed supernova in a galaxy-scale system that is suitable for time-delay cosmography studies. In this work, we present high-resolution -band adaptive optics imaging of SN Winny obtained with the near-infrared camera (NIRC2) on the W. M. Keck II telescope. With exquisite image quality (FWHM) we determine and make use of the precise astrometric positions of the five multiple images as constraints for our lens mass models. With lenstronomy and Glee, we parameterize the total mass of the system with a singular isothermal ellipsoid, a singular isothermal sphere, and external shear. The two independent models are in excellent agreement and reproduce the observed image positions with sub-milli-arcsecond residuals. The inferred projected total masses enclosed within the Einstein radii of the primary and secondary lens galaxies are M = 4.44 and M = 0.96, respectively. Likewise, the inferred effective velocity dispersion of the primary lens is 277.4 km/s, consistent with the independent spectroscopic measurement made by DESI of 298 km/s. Our modeling results are also consistent with previous results for the same system with data from the Large Binocular Telescope (LBT), using the same lens modeling codes. We also corroborate their finding that the SN multiple image A has an anomalous excess of flux by a factor of ~2-3 beyond what our smooth mass models predict.

Paper Structure

This paper contains 14 sections, 1 equation, 6 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observations & Data
  3. Keck/NIRC2
  4. PSF & Lens Light Model
  5. Multiple image E
  6. Lens modeling methodology
  7. Mass Parameterization & Priors
  8. Modeling Procedure
  9. Modeling Results
  10. Magnifications & Flux Ratios
  11. Comparison with Ecker et al. 2026
  12. Astrometric Discrepancies
  13. Model Parameters
  14. Conclusions

Figures (6)

  • Figure 1: Keck/NIRC2 imaging data of SN Winny in the K$_p$ band. Following the convention introduced by previous works, we label the two lens galaxies with G1 and G2. The multiple images of SN Winny are identified with A–E. Both images are $5.2\arcsec \times 5.2\arcsec$, North is up and East is left. Left: The fully reduced image of SN Winny with no alterations. Right: The image of SN Winny convolved with a Gaussian kernel with a $\sigma$ width equivalent to the FWHM (0065). This image also uses more dramatic clipping of pixel values.
  • Figure 2: Keck/NIRC2 imaging data (left panel), the best-fit light model from Glee (middle panel), and the normalized residual image within a range between $-3 \sigma$ and $3 \sigma$ with boxes centered on the multiple images A$-$E (right panel). The primary panels are $5.2\arcsec \times 5.2\arcsec$. North is up and East is left. Shown on the right-most end of the figure, are $0.4\arcsec \times 0.4\arcsec$ zoom-in cutouts of the normalized-residual image, centered on each of the multiple images.
  • Figure 3: Posterior distributions for the ten free parameters used in the modeling procedure for lenstronomy and Glee. Above each posterior distribution, we show the median value with the 16th and 84th percentile range. See Table \ref{['tab:priors']} for the descriptions of the parameters.
  • Figure 4: Left: Convergence map of the final lenstronomy model. The caustic (green line) and unlensed-source position (yellow star) are shown on the source-plane and the critical curve (red line), predicted (grey diamonds), and observed image positions (blue points) are shown on the lens-plane along with the light centroids (pink points) of G1 and G2. The marker size for the predicted multiple image positions is scaled with the predicted magnifications reported in Table \ref{['tab:mags']}. Right: A zoomed-in view of the caustic shape and the source position relative to the caustic edges.
  • Figure 5: Flux-to-magnification ratios for each of the five lensed images, normalized to image B with a horizontal dashed line at unity (1). We show the ratios for both the lenstronomy and Glee model predicted magnifications with 1 and $2\sigma$ intervals for each.
  • ...and 1 more figures