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Two years of shock interaction tracing three phases of evolution: the explosion of a Type IIn supernova, SN 2019vxm

This paper presents a two-year multi-wavelength study of the energetic Type IIn supernova SN 2019vxm, tracing its evolution through three distinct phases: an initial shock-interaction-dominated epoch, a transition to photon-scattering signatures, and a late stage marked by dust formation and asymmetric line profiles, while estimating a circumstellar mass of 3–8 solar masses and an ejecta mass of approximately 3.88 solar masses.

Original authors: Gitika Rameshan, Rishabh Singh Teja, D. K. Sahu, G. C. Anupama, Masayuki Yamanaka, Keiichi Maeda, Tatsuya Nakaoka, Sota Goto, Brajesh Kumar, Avinash Singh, Miho Kawabata, Koji S. Kawabata, Kenta Taguc

Published 2026-06-25

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CC BY 4.0

Original authors: Gitika Rameshan, Rishabh Singh Teja, D. K. Sahu, G. C. Anupama, Masayuki Yamanaka, Keiichi Maeda, Tatsuya Nakaoka, Sota Goto, Brajesh Kumar, Avinash Singh, Miho Kawabata, Koji S. Kawabata, Kenta Taguchi

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 Big Picture: A Star's Dramatic Finale

Imagine a massive star as a giant, over-pressurized balloon. When it runs out of fuel, it can't hold itself together anymore and collapses, triggering a massive explosion. This is a Supernova.

Most supernovas are like a firework that flashes brightly and then fades away quickly. But SN 2019vxm is different. It's a "slow-burner." This paper tracks this specific explosion for over two years, watching it evolve through three distinct "acts," much like a long movie with a slow buildup, a long middle, and a dramatic ending.

Act 1: The Slow Rise (The "Warm-Up")

When the explosion first happened, it didn't flash instantly. Instead, it took about 46 days just to reach its peak brightness in the red part of the light spectrum.

The Analogy: Think of a kettle on a stove. Most kettles boil in a minute. This one took nearly an hour to start whistling.
Why? The star didn't just explode into empty space. It was surrounded by a thick, dense cloud of gas (called Circumstellar Material or CSM) that it had coughed up before it died. The explosion had to push through this thick fog, which slowed down the initial flash.

Act 2: The Long Plateau (The "Collision Course")

Once it hit its peak, the supernova didn't fade away quickly. Instead, it stayed bright for a long time, creating a "plateau" in its light curve.

The Analogy: Imagine a car crashing into a wall of sand. The car (the explosion) doesn't stop immediately; it keeps pushing the sand forward, creating a massive pile-up. That pile-up glows because of the friction and energy of the crash.
The Science: The paper calculates that the star had dumped a huge amount of gas (between 3 to 8 times the mass of our Sun) around itself before exploding. The explosion is essentially smashing into this gas, converting the crash energy into light. This is why the supernova is so bright and lasts so long.

Act 3: The Dusty Twist (The "Red Shift")

After about a year and a half, things started to get weird.

The Light Changed: The visible light (what our eyes see) started to fade faster, but the Infrared (IR) light (heat we can't see) started to get brighter.
The Colors Shifted: If you look at the light coming from the explosion, the "red" side of the spectrum started to disappear, leaving mostly the "blue" side.
The Analogy: Imagine a bright stage light shining through a thick, red fog. At first, you see the whole beam. But as the fog gets thicker, the red light gets blocked, and you only see the blue light passing through. Also, the fog itself starts to glow with heat (Infrared).

What caused this? The paper suggests Dust.

Pre-existing Dust: The star might have had a dusty coat before it exploded, which is now being heated by the blast.
New Dust: The explosion might have created new dust grains in the cooling gas.
The Evidence: The "missing" red light and the glowing heat are classic signs of dust blocking and re-radiating energy. The paper estimates this dust is at a temperature of about 1,500 K (hot enough to be glowing, but not hot enough to melt instantly).

The Mystery of the "Invisible" Core

Usually, after a few years, the outer shell of a supernova clears out, and we can see the "core" glowing with specific chemical lines (nebular lines).

The Finding: In SN 2019vxm, even after two years, no core lines were visible.
The Analogy: It's like looking at a lighthouse through a thick, opaque wall. You can see the light hitting the wall, but you can't see the lamp inside.
Conclusion: The explosion is either still very dense, or it is completely hidden behind the thick dust and gas cloud it created.

Key Takeaways

It's a Heavy Hitter: This explosion released a massive amount of energy (500 billion billion billion joules), putting it in the top tier of energetic supernovas.
The Star Was a Messy Eater: Before it died, the star was losing mass at a crazy rate, creating a thick shell of gas and dust around it.
The "Two-Phase" Engine: The explosion was powered first by the crash into the gas (interaction), and later, as the gas cooled, by the heat from the newly formed dust.
No Precursors: Surprisingly, telescopes didn't see the star "coughing up" gas in the years immediately before the explosion. The gas was likely lost long before, or the "coughing" was too faint to see.

In short, SN 2019vxm is a cosmic crash test where a star exploded into its own thick, dusty blanket, creating a long, bright, and mysterious show that is slowly being swallowed by its own dust.

Technical Summary: Two Years of Shock Interaction Tracing Three Phases of Evolution in SN 2019vxm

Problem and Context
Type IIn supernovae (SNe) are a rare subclass of core-collapse events characterized by the interaction of ejecta with dense circumstellar material (CSM). While they are known for their high luminosity and diverse evolutionary profiles, a comprehensive understanding of their progenitors and mass-loss mechanisms requires a large sample of well-studied events. SN 2019vxm (ASASSN-19acc), discovered in December 2019, presents a unique case of a long-lived interacting supernova. Previous studies had modeled its early light curve or discussed its association with a gamma-ray burst, but a multi-wavelength analysis spanning two years post-explosion was lacking. This paper addresses the need to characterize the full evolutionary timeline of SN 2019vxm, specifically focusing on the transition from interaction-dominated phases to late-time dust signatures, and to constrain the physical properties of the ejecta and CSM.

Methodology
The authors present a comprehensive dataset comprising multi-wavelength photometry and optical spectroscopy spanning over two years (from ~16 to ~716 days post-explosion).

Observations: Optical photometry was obtained using the 2-m Himalayan Chandra Telescope (HCT) with the HFOSC instrument. Near-infrared (NIR) data were collected using the Kanata Telescope with the HONIR camera. Early-phase UV and optical data were retrieved from the Swift/UVOT archives. Pre-explosion mass-loss searches were conducted using Asteroid Terrestrial-impact Last Alert System (ATLAS) data.
Spectroscopy: Optical spectra were obtained using HCT with two grisms (Gr7 and Gr8), covering the range 3500–9250 Å.
Analysis Techniques:
- Photometry: Light curves were constructed in UVOIR bands. Pseudo-bolometric luminosities were calculated by integrating flux over 1600–23064 Å.
- Spectral Energy Distribution (SED): SEDs were modeled using single and dual blackbody components to separate the photospheric emission from potential dust emission.
- Light Curve Modeling: The UV-optical light curves were modeled using MOSFiT (a Bayesian inference code) employing the CSM interaction model to constrain CSM mass ( $M_{CSM}$ ), ejecta mass ( $M_{ej}$ ), and density profiles.
- Spectral Analysis: Line profiles (specifically H $\alpha$ and H $\beta$ ) were decomposed into narrow, intermediate, and broad components to track velocity evolution and asymmetry. Balmer decrements were calculated to infer scattering properties.

Key Results

Photometric Evolution: SN 2019vxm is a slow-rising ( $\sim$ $\sim$ 45.9 days in R-band) and slow-declining supernova, reaching a peak absolute magnitude of $\approx$ $\approx$ -20.3 mag. The total radiated energy is estimated at $5.23 \times 10^{50}$ $5.23 \times 1 0^{50}$ erg, placing it among the energetic Type IIn events. The light curve exhibits three distinct phases:
- Rise: Dominated by UV emission.
- Shallow Decline: A plateau-like phase (up to ~400 days) driven by CSM interaction.
- Steep Decline: A post-400 day phase where the optical flux drops rapidly while IR emission brightens significantly, indicating a transition to an IR-dominated regime.
Physical Parameters:
- CSM Mass: Light curve modeling (independent of specific density profile assumptions) and comparison with theoretical radiative transfer models (HERACLES) suggest a massive CSM of 3–8 $M_\odot$ .
- Ejecta Mass: Derived from the broad H $\alpha$ component flux, the minimum ejecta mass is estimated at $\sim$ 3.88 $M_\odot$ . This is consistent with light curve models but lower than some previous estimates, suggesting significant stripping of the progenitor envelope.
- Dust: SED modeling at late epochs ( $t > 400$ days) reveals an IR excess best fit by a dust component with temperatures of $\sim$ 1500 K located at a radius of $\sim$ 4 $\times$ 10 $^{16}$ cm.
Spectral Evolution and Asymmetry:
- Early Phase: Spectra are dominated by narrow and intermediate emission lines from the CSM and Cold Dense Shell (CDS), with symmetric profiles.
- Intermediate Phase: Asymmetric line profiles emerge, characterized by symmetric wings but a suppression of red flux. This asymmetry is attributed to photon scattering by an optically thick medium.
- Late Phase: The red side of the H $\alpha$ line is strongly suppressed, and the profile becomes increasingly blueshifted. The Balmer decrement increases significantly ( $\sim$ 10), indicating wavelength-dependent extinction.
- Nebular Lines: No nebular lines are detected even at late epochs, implying the ejecta remain dense or are heavily obscured.

Significance and Claims
The paper claims that SN 2019vxm serves as a critical case study for understanding the late-time evolution of interacting supernovae. The authors highlight three specific contributions:

Evolutionary Phases: The study clearly delineates three evolutionary phases, demonstrating that the transition from an interaction-dominated optical plateau to an IR-dominated decline is a distinct feature of this object, likely driven by the emergence of dust.
Dust Influence: The combination of optical fading, IR brightening, and the specific nature of the line asymmetry (red flux suppression) provides strong evidence for the presence of dust. The authors argue this dust could be either pre-existing in the CSM or newly formed, though the high photospheric temperature ( $\sim$ 5000 K) makes in-situ dust formation in the inner ejecta unlikely at the observed epoch.
Progenitor Constraints: The derived CSM mass of 3–8 $M_\odot$ and ejecta mass of $\sim$ 3.88 $M_\odot$ suggest a massive progenitor (likely $\sim$ 40 $M_\odot$ ) that underwent significant mass loss prior to explosion, potentially through eruptive mechanisms or binary interaction, though no precursor outbursts were detected in the 4 years prior to discovery.

The authors conclude that while the exact origin of the dust (pre-existing vs. newly formed) cannot be definitively distinguished with current data, the observed phenomena are consistent with a dense, dusty environment surrounding the supernova, which obscures the ejecta and alters the observed spectral energy distribution.