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

This paper presents high-resolution Keck/NIRC2 adaptive optics imaging and preliminary lens modeling of SN 2025wny, the first gravitationally lensed Type I superluminous supernova, which successfully constrains the masses of the lensing galaxies and confirms an anomalous flux excess in one of the multiple images.

The Gist

Imagine looking up at the night sky and seeing a single, brilliant star that suddenly splits into five distinct copies of itself, arranged in a perfect cosmic pattern. This isn't a trick of the eye or a glitch in the camera; it's a cosmic magic trick performed by gravity itself.

This paper is about Supernova 2025wny (affectionately nicknamed "SN Winny"), a spectacular stellar explosion that was caught in the act of being duplicated by a massive cosmic lens. Here is the story of how astronomers caught this rare event and what it tells us about the universe.

1. The Cosmic Camera Trap

Think of gravity not just as a force that pulls things down, but as a giant, invisible magnifying glass made of dark matter and stars. When a massive object (like a pair of galaxies) sits between us and a distant explosion, its gravity bends the light coming from that explosion.

Usually, this bending creates a blurry smear. But in the case of SN Winny, the alignment was perfect. The gravity of two foreground galaxies acted like a high-end camera lens, splitting the light from the exploding star into five distinct images (labeled A, B, C, D, and E).

This is incredibly rare. It's like finding a single raindrop that has been perfectly split into five identical droplets by a specific arrangement of rocks in a stream. SN Winny is the first time we've seen this happen with a "Superluminous Supernova" (a star that explodes with the brightness of a billion suns) in a system small enough to study in detail.

2. The High-Definition Snapshot

To understand how this lens works, you need a really sharp picture. The team used the Keck II Telescope in Hawaii, equipped with a special "adaptive optics" system.

• The Analogy: Imagine trying to take a photo of a firefly through a wavy, heat-distorted window. The image would be blurry. Adaptive optics is like a magical window that instantly smooths out the ripples in the air (caused by Earth's atmosphere) to give you a crystal-clear view.
• The Result: The team got an image so sharp it's like looking at a coin from 100 miles away. This allowed them to pinpoint the exact location of the five ghostly images of the supernova with incredible precision.

3. The Detective Work: Solving the Puzzle

Now that they had the picture, the astronomers had to figure out the shape of the "lens" (the two galaxies) that caused the split. They used two different super-computer programs, lenstronomy and Glee, to act as digital detectives.

• The Goal: They wanted to build a 3D map of the invisible mass of the two galaxies.
• The Method: They treated the five images of the supernova like clues on a treasure map. By working backward from where the images appeared, they calculated how much mass the galaxies must have to bend the light that way.
• The Check: They ran the puzzle through two different software programs. If both programs solved the puzzle and gave the same answer, they knew they were right. And they did! Both programs agreed perfectly, confirming the mass of the galaxies and their shapes.

4. The Mystery of the "Extra Bright" Ghost

Here is where things get interesting. When the astronomers compared the brightness of the five images to what their computer models predicted, they found a glitch.

• The Expectation: The models predicted that Image A should be a certain brightness.
• The Reality: Image A was 2 to 3 times brighter than the model said it should be.
• The Analogy: Imagine you are looking at a reflection in a pond. You expect the reflection to be a certain size. Suddenly, you notice one reflection is glowing like a neon sign, while the others are normal.
• The Cause: This "anomalous brightness" suggests that there are tiny, invisible clumps of dark matter or other stars sitting right in the path of the light for Image A, acting like tiny magnifying glasses that boost its brightness. It's a clue that the universe is full of hidden, small-scale structures we can't see directly.

5. Why Does This Matter?

You might ask, "Why do we care about a split star?"

This system is a cosmic stopwatch. Because the light takes different paths to reach us, the five images of the supernova don't all light up at the same time. Some arrive a few days earlier than others.

• The Hubble Constant: By measuring exactly how long the delay is between the images, astronomers can calculate the Hubble Constant ($H_0$). This is a number that tells us how fast the universe is expanding.
• The Conflict: Currently, there is a major disagreement in physics about how fast the universe is expanding. Different methods give different answers. SN Winny offers a brand-new, independent way to measure this speed, potentially solving one of the biggest mysteries in modern physics.

The Bottom Line

This paper is a "proof of concept." It shows that we can now catch these rare, galaxy-scale lensed supernovae, map their invisible lenses with high precision, and use them as tools to measure the universe.

SN Winny is the first of many. As new telescopes come online, we expect to find hundreds more of these "cosmic split-stars." Each one is a new key that will help us unlock the secrets of dark matter, dark energy, and the ultimate fate of our universe. It's the beginning of a new era where we don't just watch the universe; we use its own gravity as a laboratory to test the laws of physics.

Technical Summary

1. Problem and Scientific Context

The Challenge: Strongly gravitationally lensed supernovae (SNe) are rare but powerful tools for "time-delay cosmography," a method to measure the Hubble constant ($H_0$) independently of the cosmic distance ladder. While lensed quasars are common, lensed SNe are scarce. Previous galaxy-scale lensed SNe (e.g., Refsdal, SN H0pe) suffered from time delays that were either too short or too complex for high-precision cosmological measurements.

The Target: SN 2025wny ("Winny") is the first discovered gravitationally lensed Type I superluminous supernova (SLSN) and the first galaxy-scale lensed SN suitable for time-delay cosmography. It is lensed by two foreground galaxies ($G1$ and $G2$) at $z=0.375$ into five multiple images (A–E). The source is at $z=2.008$.

The Gap: While previous studies (e.g., Ecker et al. 2026, "E26") used Large Binocular Telescope (LBT) data, there was a need for higher-resolution imaging to precisely constrain the astrometric positions of the multiple images and the lens galaxies, which are critical for accurate mass modeling and time-delay predictions.

2. Methodology

Observations:

• Instrument: W. M. Keck II telescope with the Near-infrared Camera, Second Generation (NIRC2).
• Technique: Laser Guide Star (LGS) Adaptive Optics (AO) in the $K_p$-band (2.12 $\mu$m).
• Data Quality: Achieved an exceptional Full Width at Half Maximum (FWHM) of $\approx 0.065''$ with a pixel scale of $0.009952''$.
• Strategy: Target of Opportunity (ToO) observations on November 11, 2025. Used "on-off" dithers to obtain dedicated sky frames and tip-tilt star observations to construct an empirical Point Spread Function (ePSF).

Data Reduction:

• Processed using the nirc2_reduce pipeline.
• Corrected for geometric distortion (Service et al. 2016) and orientation offsets ($0.252^\circ$).
• Constructed an ePSF using the PSFr algorithm on tip-tilt star images.

Lens Modeling:
The authors employed two independent, state-of-the-art lens modeling codes to ensure robustness:

1. lenstronomy: Uses a source-plane likelihood approach with Particle Swarm Optimization (PSO) and MCMC.
2. Glee: Uses an image-plane likelihood approach with MCMC.

Mass Parameterization:

• Lens Galaxies: $G1$ modeled as a Singular Isothermal Ellipsoid (SIE); $G2$ modeled as a Singular Isothermal Sphere (SIS).
• External Shear: Included to account for line-of-sight structure.
• Constraints: The five observed image positions (A–E) serve as the primary observables.
• Priors: Mass centroids were constrained by the light centroids of the lens galaxies (derived from S'ersic profile fitting).
• Multiplicity Constraint: A specific constraint was applied to ensure the model predicts exactly five images (excluding the theoretical "on-core" images), preventing the source from falling into an "inner caustic" region that would produce seven images.

3. Key Contributions

1. Highest Resolution Imaging: Provided the first high-angular resolution ($0.065''$) $K_p$-band imaging of a galaxy-scale lensed SN, enabling sub-milli-arcsecond astrometry.
2. Discovery of Image E: Confirmed the detection of the fifth multiple image (E) at $\sim 7.5\sigma$, located $\sim 0.2''$ from the secondary lens galaxy ($G2$).
3. Dual-Code Validation: Demonstrated excellent agreement between two independent modeling frameworks (lenstronomy and Glee), validating the robustness of the derived physical parameters.
4. Flux Anomaly Quantification: Confirmed that Image A exhibits a flux excess of $\sim 2-3\times$ over smooth mass model predictions, likely due to microlensing or substructure, while Images B and D are consistent with models.
5. Cross-Instrument Comparison: Conducted a rigorous comparison between Keck/NIRC2 and LBT/LUCI data, identifying a systematic scale difference of $\sim 0.6\%$ and a rotation offset of $\sim 0.62^\circ$, while confirming that intrinsic mass parameters remain consistent within $2\sigma$.

4. Key Results

Astrometry and Mass Modeling:

• Image Positions: The models reproduce the observed positions of the five images with residuals $< 0.004''$.
• Einstein Radii:
  • Primary Lens ($G1$): $\theta_{E,1} \approx 1.58''$
  • Secondary Lens ($G2$): $\theta_{E,2} \approx 0.74''$
• Masses:
  • Enclosed mass in $G1$: $M_1 = 4.44^{+0.06}_{-0.05} \times 10^{11} M_\odot$
  • Enclosed mass in $G2$: $M_2 = 0.96^{+0.02}_{-0.02} \times 10^{11} M_\odot$
• Velocity Dispersion:
  • Inferred effective velocity dispersion for $G1$: $\sigma_1 = 277.4^{+0.9}_{-0.7}$ km/s.
  • This is consistent with the independent DESI spectroscopic measurement of $\sigma_{\star,1} = 298 \pm 37$ km/s.

Magnifications and Flux:

• Magnification: Predicted magnifications range from $\mu \approx -1.6$ (Image E) to $\mu \approx -8.8$ (Image A).
• Flux Anomaly: Image A is anomalously bright by a factor of $\sim 2-3$ compared to the smooth model. This is attributed to microlensing by stars or milli-lensing by dark matter subhalos.
• Time Delays: While not measured in this paper, the authors note that Images A, C, and E are roughly in-phase (delays of a few days), suggesting the flux anomaly is astrophysical rather than a phase difference in the SN light curve.

Systematics:

• A comparison with E26 (LBT data) revealed a residual RMS of $\sim 5$ mas after correcting for translation, rotation, and scale. The scale difference ($\sim 0.6\%$) explains the discrepancy in the Einstein radius ($\sim 1.6\sigma$) between the two datasets.

5. Significance

• Cosmological Potential: SN 2025wny is the first galaxy-scale lensed SN with time delays suitable for high-precision $H_0$ measurements. The precise mass models derived here are the necessary foundation for future time-delay measurements.
• Methodological Benchmark: The paper establishes a robust workflow for modeling galaxy-scale lensed SNe, combining high-resolution AO imaging with dual-code verification. This framework will be critical for the upcoming flood of lensed SNe expected from the Vera C. Rubin Observatory (LSST) and the Roman Space Telescope.
• Substructure Probes: The confirmed flux anomaly in Image A provides a new avenue for probing dark matter substructure and microlensing populations in lens galaxies, complementing quasar studies.
• Era of Precision: SN 2025wny marks the transition from the discovery phase of lensed SNe to the era of precision cosmology, paving the way for a potential $1\%$ measurement of the Hubble constant using strong lensing.