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VENUS: Strong-lensing model of MACS J1931.8-2635 -- revealing the farthest multiply imaged supernova

This paper presents a high-precision strong-lensing model of the galaxy cluster MACS J1931.8-2635, refined with 19 multiple-image systems including 10 new identifications from VENUS JWST imaging, to characterize the magnification and time delays of the five predicted images of the distant supernova SN Eos at $z=5.13$ and demonstrate the potential of such high-redshift lensed supernovae for cosmography.

Original authors: Joseph F. V. Allingham, Adi Zitrin, Vasily Kokorev, Hiroto Yanagisawa, Jose M. Diego, Lukas J. Furtak, Yoshihisa Asada, Dan Coe, David A. Coulter, Seiji Fujimoto, Conor Larison, Masamune Oguri, Justin

Published 2026-07-08

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CC BY 4.0

Original authors: Joseph F. V. Allingham, Adi Zitrin, Vasily Kokorev, Hiroto Yanagisawa, Jose M. Diego, Lukas J. Furtak, Yoshihisa Asada, Dan Coe, David A. Coulter, Seiji Fujimoto, Conor Larison, Masamune Oguri, Justin D. R. Pierel, Fengwu Sun, Marusa Bradac, Pratika Dayal, Paulo A. A. Lopes, Ashish K. Meena, Massimo Pascale, Hollis B. Akins, Franz E. Bauer, Larry D. Bradley, Gabriel Brammer, John Chisholm, Guillaume Desprez, Qinyue Fei, Henry C. Ferguson, Steven L. Finkelstein, Brenda Frye, Miriam Golubchik, Kohei Inayoshi, Yolanda Jimenez-Teja, Anton M. Koekemoer, Ray A. Lucas, Georgios E. Magdis, Nicholas S. Martis, Richard Pan, Johan Richard, Massimo Ricotti, Gregor Rihtarsic, Luke Robbins, William Sheu, Brian Welch, Chris Willott, Rogier A. Windhorst

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Magnifying Glass

Imagine the universe is filled with massive, invisible structures called galaxy clusters. These clusters are so heavy that they warp the fabric of space-time around them, acting like a giant, natural magnifying glass. This phenomenon is called "gravitational lensing."

When light from a very distant, faint object (like a galaxy or a star) passes behind one of these clusters, the cluster bends that light. This does two amazing things:

It magnifies the object: It makes faint things look much brighter, allowing us to see things that would otherwise be invisible.
It multiplies the image: It can split the light into multiple copies, so we see the same object in several different places in the sky at once.

The Discovery: A Supernova in the Distance

The authors of this paper studied a specific galaxy cluster named MACS J1931.8-2635. Using the powerful James Webb Space Telescope (JWST), they took incredibly sharp photos of this cluster.

In these photos, they spotted two tiny, red dots right next to the cluster's central galaxy. At first glance, they looked like two separate stars. However, the team built a detailed mathematical map (a lens model) of the cluster's gravity to understand how it bends light.

The map revealed a surprise: those two red dots weren't two different objects. They were actually two images of the exact same object, seen from different angles because the cluster's gravity bent the light around it.

Even more exciting, the team realized this object was a supernova (an exploding star) named SN Eos.

How far away? It is incredibly far, at a distance corresponding to a redshift of 5.13. This makes it the farthest multiply-imaged supernova ever discovered.
Why did they know it was a supernova? The mathematical model predicted there should be five images of this supernova in total. The team saw two. They looked for the other three predicted images but found nothing there. This "missing" light confirmed the object was a transient—something that appears and disappears quickly (like an explosion) rather than a permanent star.

The "Time Travel" Aspect

One of the most fascinating parts of this discovery is time delay. Because the light takes different paths to get to us around the cluster, the images arrive at different times.

The "Fast" Images: The two images the team saw (SN Eos) arrived almost at the same time (within 5 days of each other).
The "Slow" Images: The model predicts that three other images of this same explosion actually arrived years ago.
- Two images arrived about 3.5 years before the ones we saw.
- One image arrived a massive 54 years before the ones we saw.

Think of it like a race where runners take different routes around a mountain. The runners who took the short, direct path finished today. The runners who took the long, winding path finished years ago. If we had been watching the sky 54 years ago, we would have seen this explosion then, too.

Other Cosmic Treasures

While the supernova was the headline, the team's new map of the galaxy cluster helped them find other hidden gems:

Charybdis: They found a beautiful, spiral-shaped galaxy that is being stretched into three images by the cluster. Because the cluster magnified it so much, they could see details on this galaxy as small as a few hundred light-years across, even though it is billions of light-years away. They named it "Charybdis" after a sea monster from Greek mythology, likely because the galaxy looks like it's swirling in a cosmic whirlpool.

Why This Matters

This paper is like a guidebook for the universe. By creating a precise map of how this specific galaxy cluster bends light, the scientists can:

Find the invisible: Use the cluster as a telescope to see the very first stars and galaxies in the universe.
Measure the universe: By studying the time delays between the different images of supernovae, scientists can calculate how fast the universe is expanding (the Hubble constant). The paper suggests that finding more of these distant, time-delayed explosions could help us measure the universe's expansion rate with greater precision.

Summary

In short, the team used the James Webb Space Telescope to look at a massive galaxy cluster. They built a gravity map that acted like a lens, revealing a distant exploding star (SN Eos) that is so far away it is the most distant one of its kind ever found. The map showed that we are seeing two "echoes" of the explosion happening now, while three other "echoes" happened decades ago. This discovery proves that these cosmic magnifying glasses are powerful tools for exploring the deepest, earliest corners of our universe.

Technical Summary: VENUS: Strong-lensing model of MACS J1931.8-2635 – revealing the farthest multiply imaged supernova

Problem and Context
Galaxy clusters serve as natural telescopes, magnifying background sources and producing multiple images with measurable time delays. While strong lensing has been instrumental in studying high-redshift galaxies, stars, and supernovae (SNe), precise lens models are required to interpret the intrinsic properties of these sources. The galaxy cluster MACS J1931.8-2635 ( $z_l = 0.35$ ) was previously modeled using CLASH survey data, but the discovery of a new transient, SN Eos, at a spectroscopic redshift of $z = 5.13$ necessitated an updated, high-precision model. The primary challenge was to construct a robust lens model capable of predicting the positions, magnifications, and time delays of the multiple images of this distant supernova and its host galaxy, while simultaneously refining the mass distribution of the cluster using newly identified multiple-image systems from recent JWST observations.

Methodology
The authors present a new parametric strong-lensing model for MACS J1931.8-2635, utilizing a combination of datasets:

Imaging: New 10-band NIRCam imaging from the VENUS survey (JWST) collected in September 2025, supplemented by archival HST/ACS and WFC3 data from the CLASH survey and SNAP program.
Spectroscopy: VLT/MUSE integral field unit data and JWST/NIRSpec spectroscopy (DDT proposal 9493) confirming the redshift of the SN Eos host galaxy ( $z = 5.133 \pm 0.001$ ) and the supernova itself.
Software: The modeling employs AstroLensPy, a new Python implementation of the ZitrinAnalytic (PIEMDeNFW) method. This code uses Numba for optimization and emcee for MCMC sampling.
Model Structure: The mass distribution is modeled using a single large-scale dark matter halo (DMH) described by a Pseudo-Isothermal Elliptical Matter Distribution (PIEMD) and a Brightest Cluster Galaxy (BCG) component. Individual cluster member galaxies are modeled as a collection of dual Pseudo-Isothermal Elliptical (dPIE) profiles, scaled via the Faber-Jackson relation, with the BCG's luminosity and ellipticity allowed to vary independently.
Constraints: The model is constrained by 19 multiply-imaged systems (51 total images), including 10 newly identified systems from VENUS data. Nine systems have spectroscopic redshifts. The redshifts of the remaining systems are treated as free parameters in the optimization, guided by photometric redshifts. The optimization minimizes $\chi^2$ in the source plane, with positional uncertainties set to $0.2''$ generally and $0.02''$ for the SN Eos images to prioritize their prediction accuracy.

Key Contributions and Results

Refined Lens Model: The final model achieves a root-mean-square (r.m.s.) deviation of $0.44''$ between observed and predicted image positions across 19 systems. This represents an improvement over previous models (e.g., $0.77''$ for the parametric CLASH model) despite incorporating 12 additional constraints and fewer degrees of freedom regarding the dark matter halo structure (using a single DMH).
SN Eos Characterization: The model predicts five images for SN Eos (System 101) and its host galaxy (System 100).
- Two images (101.1 and 101.2) were observed simultaneously in VENUS data. The model predicts a magnification of $\mu \simeq 25-30$ for this pair and a time delay of $< 5$ days, consistent with their simultaneous detection.
- Three additional images (p101.3, p101.4, p101.5) are predicted to have arrived earlier, with time delays of $3.6 \pm 0.7$ , $3.4 \pm 0.7$ , and $53.9 \pm 10.8$ years, respectively. The non-detection of these earlier images in archival HST data confirms the transient nature of the source.
- The host galaxy (System 100) is detected in MUSE data with four images; the fifth predicted image lies outside the MUSE field of view.
Discovery of "Charybdis": The lens model identifies a triply-imaged, grand-design spiral galaxy candidate (System 14, named Charybdis) at $z \simeq 3.65$ . The model reconstructs the source with a resolution of $\simeq 90$ pc in the source plane. Spectral Energy Distribution (SED) fitting yields a stellar mass of $\log_{10}(M_*/M_\odot) = 10.64 \pm 0.24$ and a star formation rate of $48.5 \pm 11.5 M_\odot \text{yr}^{-1}$ .
Cosmological Implications: The authors investigate the utility of high-redshift lensed SNe for cosmography.
- Hubble Constant ( $H_0$ ): Simulations indicate that time delays generally increase with source redshift, suggesting that higher-redshift sources like SN Eos could provide tighter constraints on $H_0$ .
- Other Parameters: For other cosmological parameters (e.g., matter density $\Omega_m$ , dark energy equation of state $w_0$ ), the study finds that lower-redshift sources may offer moderately stronger constraints in specific cases, though high-redshift sources still retain significant discriminating power ( $\sim 5\%$ variation in time-delay distance).

Significance and Claims
The paper claims that the detection of SN Eos, the farthest multiply-lensed supernova to date, was facilitated by the lens model itself, which identified the source as a transient due to the simultaneous observation of two images with a short predicted time delay and the absence of earlier images. The study demonstrates that combining deep JWST imaging with robust parametric lens modeling allows for the precise characterization of high-redshift transients and their hosts.

The authors emphasize that while high-redshift lensed SNe are promising for constraining $H_0$ due to larger time delays, their utility for constraining other cosmological parameters requires careful consideration of the specific lens model and source redshift. The work highlights the potential of regular monitoring of strong-lensing clusters to yield substantial samples of multiply-imaged transients, enabling future cosmographic studies based on time delays and distance ratios. The lens model and associated catalogs are made publicly available to support further research into the cluster's mass distribution and the properties of its background sources.