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The Eye of Sauron in SN 2025ngs: a Short-plateau Cousin of SN 1998S with Evidence for a Ring-like Circumstellar Medium

This paper presents SN 2025ngs, a peculiar interacting supernova in NGC 5961 that exhibits a short-plateau light curve and spectral features resembling SN 1998S, providing strong evidence for a progenitor with a high mass-loss rate surrounded by a ring-like circumstellar medium.

Original authors: Conor L. Ransome, David J. Sand, K. Azalee Bostroem, Aravind P. Ravi, Bhagya M. Subrayan, Jennifer E. Andrews, Zachary G. Lane, Yize Dong, Anya Nugent, Stefano Valenti, Jeniveve Pearson, Manisha Shres

Published 2026-05-08

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CC BY 4.0

Original authors: Conor L. Ransome, David J. Sand, K. Azalee Bostroem, Aravind P. Ravi, Bhagya M. Subrayan, Jennifer E. Andrews, Zachary G. Lane, Yize Dong, Anya Nugent, Stefano Valenti, Jeniveve Pearson, Manisha Shrestha, Samaporn Tinyanont, Brian Hsu, Moira Andrews, Dominik Banhidi, Imre Barna Biro, Collin Christy, Istvan Csanyi, Joseph Farah, Noah Franz, Emily T. Hoang, Griffin Hosseinzadeh, D. Andrew Howell, Daryl Janzen, Saurabh W. Jha, Lindsey A. Kwok, Chang Liu, Michael Lundquist, Aidan Martas, Curtis McCully, Darshana Mehta, Nicolas E. Meza Retamal, Nathan Smith, Tamas Szalai, Sergiy Vasylyev, V. Ashley Villar, Kathryn Wynn

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 Eye of Sauron in the Sky: A Star's Dramatic Final Act

Imagine a massive star, the size of our sun but hundreds of times bigger, living its final days. Usually, when such a star runs out of fuel, it collapses and explodes in a spectacular supernova. But sometimes, the story is more complicated. The star doesn't just die; it throws a tantrum first, shedding layers of its own skin in a violent storm before the final explosion.

This is the story of SN 2025ngs, a cosmic event discovered in a galaxy called NGC 5961, about 120 million light-years away. Astronomers have nicknamed this event "The Eye of Sauron" because of a unique feature in its light, reminiscent of the fiery, ringed eye from The Lord of the Rings.

Here is the story of what happened, explained simply:

1. The "Flash" Before the Storm

When the star finally exploded, it didn't just go "boom" and fade away. Instead, the explosion hit a thick, dense cloud of gas that the star had spewed out just days or weeks before it died.

Think of it like a car crashing into a pile of snow it had just dumped on the road. The impact creates a bright, sudden flash. In astronomy, we call this a "flash supernova." For about a week, SN 2025ngs showed bright, narrow lines in its light spectrum, proving it was smashing into this nearby cloud.

2. The "Short Plateau"

Most stars of this type (called Type II supernovae) have a light curve that looks like a long, flat table. After the initial flash, they stay bright for about 100 days before slowly fading. This is called a "plateau."

SN 2025ngs was different. It was a "short-plateau" supernova. It stayed bright for only about 70 days before starting to drop off. It's like a candle that burns steadily for a while but then runs out of wax much faster than expected. This suggests the star didn't have as much "fuel" (hydrogen) left as its slower-burning cousins.

3. The "Eye of Sauron" Reappears

Here is the most magical part. As the supernova began to fade after those 70 days, something strange happened. The light didn't just get dimmer; the shape of the light changed.

Astronomers looked at the hydrogen in the explosion and saw a very specific shape: a double-horned profile. Imagine looking at a mountain range with two distinct peaks and a valley in the middle. In the paper, the authors compare this to the "Eye of Sauron"—a bright ring with a dark center.

This shape is the smoking gun. It tells us that the star didn't just blow gas out in a sphere (like a balloon). Instead, it blew gas out in a flat disk or a ring, like a hula hoop spinning around the star. When the explosion hit this ring, it created that double-peaked "eye" shape. This is rare evidence that the star was likely spinning fast or had a partner star that helped shape the gas into a ring before the explosion.

4. The "Ghost" of SN 1998S

SN 2025ngs looks a lot like a famous supernova from 1998 called SN 1998S. They are like distant cousins.

The Similarity: They both have the same weird, complex "ring" shape in their light and the same messy history of shedding gas.
The Difference: SN 1998S was much brighter and faded in a straight line (like a ramp), while SN 2025ngs was dimmer and had that short, flat plateau.

This tells us that even though these stars look different in brightness, they might be going through the same messy, violent process of dying.

5. What Was the Star Like?

Based on the clues, the star that exploded was likely a Red Supergiant (a huge, cool, red star).

The Mass Loss: Before it died, this star was losing mass at a rate thousands of times faster than our sun loses mass. It was coughing up a massive amount of gas, creating the ring that caused the "Eye of Sauron" effect.
The Binary Theory: The paper suggests this star probably had a "partner" (a binary companion). Just like two dancers spinning, the gravity of the partner likely helped strip the star's skin and shape the gas into that flat ring.

Summary

SN 2025ngs is a cosmic detective story. By looking at how the light changed over time, astronomers figured out that:

The star exploded into a ring of gas it had just created.
This ring created a unique "double-horned" light signature, earning it the "Eye of Sauron" nickname.
It is a "short-plateau" supernova, meaning it burned out faster than most.
It bridges the gap between normal exploding stars and the most extreme, interacting ones.

It's a reminder that when massive stars die, they don't just vanish; they leave behind a complex, sculpted graveyard of gas that tells the story of their final, chaotic moments.

Technical Summary: The Eye of Sauron in SN 2025ngs

Problem and Context
Core-collapse supernovae (CCSNe) provide a critical window into the final evolutionary stages of massive stars. While Type IIP supernovae are typically associated with red supergiant (RSG) progenitors retaining a hydrogen envelope, and Type IIn supernovae exhibit strong interaction with dense circumstellar material (CSM), a class of "transitional" objects exists that bridges these categories. These objects, such as SN 1998S, display early "flash" features indicative of confined CSM interaction but evolve into spectra resembling standard Type II events. A significant gap remains in understanding the mass-loss mechanisms (e.g., binary interactions, unstable burning) that produce such complex CSM geometries, particularly disk-like or ring-like structures, and how these influence the photometric and spectroscopic evolution of the transient.

Methodology
The authors present a comprehensive multi-wavelength study of SN 2025ngs, discovered in NGC 5961 (37.8 Mpc) by ATLAS on June 12, 2025. The study utilizes:

Photometry: High-cadence observations from DLT40, Las Cumbres Observatory (LCO), ATLAS, and Swift (UV/optical) spanning from pre-explosion limits to ~120 days post-explosion.
Spectroscopy: A dense time-series of optical spectra from Gemini-N (GMOS), MMT (Binospec), LCO (FLOYDS), and Keck (MOSFIRE), alongside near-infrared (NIR) data from IRTF and MMT (MMIRS).
High-Resolution Spectroscopy: Early-time (2–3 days post-explosion) high-resolution ( $R \approx 80,000$ ) spectra obtained with MAROON-X on Gemini-N to resolve narrow CSM features and host extinction.
Analysis Techniques:
- Extinction: Derived host reddening ( $E(B-V)_{host}$ ) via Markov-Chain Monte Carlo (MCMC) fitting of Na I D absorption lines in the MAROON-X spectra.
- Light Curve Modeling: Estimation of explosion time via parabolic rise fitting; pseudo-bolometric light curve construction; and fitting of early light curves to shock-cooling models (J. Morag et al. 2023).
- Spectral Modeling: Comparison of early spectra to CMFGEN models (I. Boian & J. H. Groh 2019; L. Dessart et al. 2017) to constrain progenitor abundances and mass-loss rates.
- Host Characterization: Analysis of the host galaxy NGC 5961 using the FrankenBlast pipeline to fit stellar population models to multi-band photometry.

Key Results

Photometric Evolution: SN 2025ngs is a luminous ( $M_V = -17.91$ mag) "short-plateau" supernova with a plateau duration of $\sim 70$ days, significantly shorter than the canonical $\sim 100$ days for Type IIP. The post-plateau decline is shallower than expected for pure radioactive decay ($0.0058$ mag day $^{-1}$ vs. $0.0098$ mag day $^{-1}$ ), suggesting ongoing CSM interaction powers the late-time luminosity. The inferred upper limit for the $^{56}$ Ni mass is $0.007 M_\odot$ .
Spectroscopic Evolution:
- Early Phase: The transient exhibited "flash" features (high-ionization C IV, N IV, He II) within the first week, indicative of interaction with dense, confined CSM.
- High-Resolution Anomaly: The MAROON-X spectrum taken $\sim 2.6$ days post-explosion revealed a unique, symmetric, double-horned H $\alpha$ profile with peaks offset by $\pm 20$ km s $^{-1}$ . This morphology is interpreted as the shock interacting with a confined, disk-like or ring-like CSM structure. This feature faded within a day, constraining the extent of the disk to $\sim 10^{14}$ cm.
- Late Phase: As the photosphere receded, the spectrum evolved to resemble SN 1998S, exhibiting complex H $\alpha$ profiles with a "blue shoulder" and persistent narrow cores, rather than standard broad P-Cygni profiles.
Progenitor Constraints: Spectral modeling suggests a progenitor with a high mass-loss rate of $\dot{M} \approx 10^{-3} M_\odot$ yr $^{-1}$ and abundances consistent with a massive RSG or Yellow Hypergiant (YHG). The low inferred nickel mass and short plateau suggest a progenitor that was partially stripped, likely via binary interaction, retaining enough hydrogen for a plateau but not enough for a long duration.
Host Galaxy: NGC 5961 is a low-mass ( $\log(M_*/M_\odot) \approx 9.37$ ), star-forming spiral galaxy with sub-solar to solar metallicity. The specific star-formation rate (SSFR) is consistent with typical SN II and SN IIn hosts.

Significance and Claims
The paper positions SN 2025ngs as a "spectroscopic mimic" of the luminous SN 1998S, despite having a distinct, shorter-plateau photometric evolution. The authors claim that SN 2025ngs represents a crucial link in a continuum between normal flash-supernovae (like SN 2023ixf) and strongly interacting Type IIn events.

The primary significance lies in the direct spectroscopic evidence for a ring-like or disk-like CSM (the "Eye of Sauron" analogy) surrounding the progenitor. The double-horned H $\alpha$ profile provides strong constraints on the geometry of mass loss shortly before the explosion, supporting scenarios involving binary interactions or unstable burning that create non-spherical CSM. The paper argues that the diversity in the "98S-like" group (spectroscopically similar but photometrically diverse) may be driven by viewing angles of these asymmetric CSM structures. Finally, the study reinforces the hypothesis that short-plateau SNe and Type IIL SNe may exist on a continuum defined by envelope mass and CSM interaction, rather than being distinct classes.