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SN2023ixf: Radiative-transfer modeling of the photospheric phase evolution from the ultraviolet to the infrared

This paper presents non-LTE time-dependent radiative-transfer modeling of SN2023ixf's photospheric phase from UV to IR, demonstrating that a 15 solar mass progenitor with enhanced mass loss and prolonged interaction with a decreasing circumstellar density shell successfully reproduces the supernova's observed brightness, spectral features, and infrared flux while revealing the emergence of a cold dense shell and the impact of asymmetry and clumping on spectral profiles.

Original authors: Luc Dessart, Wynn V. Jacobson-Galan, K. Azalee Bostroem, Alexei V. Filippenko, WeiKang Zheng, Thomas G. Brink, Stefano Valenti

Published 2026-04-08

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CC BY 4.0

Original authors: Luc Dessart, Wynn V. Jacobson-Galan, K. Azalee Bostroem, Alexei V. Filippenko, WeiKang Zheng, Thomas G. Brink, Stefano Valenti

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

Imagine a supernova, SN 2023ixf, as a massive cosmic firework exploding in a nearby galaxy. But this wasn't just a simple boom; it was a complex, multi-layered event where the explosion had to push through a thick, messy fog of gas left behind by the dying star before it could shine brightly.

This paper is like a high-tech detective story. The authors, led by Luc Dessart, used supercomputers to build a "virtual twin" of this explosion to understand exactly what happened between 20 and 120 days after the blast. They wanted to see how the light traveled from the ultraviolet (invisible to our eyes) to the infrared (heat radiation), covering the whole electromagnetic spectrum.

Here is the story of their findings, explained with some everyday analogies:

1. The Star and the "Fog"

The star that exploded was a Red Supergiant—a giant, bloated sun that had been shedding its skin (gas) for a long time. Think of it like a person blowing out a lot of breath before sneezing. When the star finally exploded, the shockwave hit this "breath" (called Circumstellar Material or CSM).

The Analogy: Imagine a car speeding through a thick fog bank. The car (the explosion) is fast, but the fog (the gas) slows it down and creates a pile-up right at the front.
The Finding: The authors found that the explosion didn't just happen in a vacuum. It was constantly bumping into this fog. This collision created a Cold Dense Shell (CDS)—a thick, compressed wall of gas right at the front of the explosion, moving at about 8,000 km/s.

2. The "Engine" of the Explosion

To match what we saw in the sky, the computer model needed a specific recipe:

The Star: A 15-sun-mass star that had lost a lot of weight before dying.
The Debris: The explosion threw out about 7–8 suns' worth of material.
The Fuel: It had a specific amount of radioactive "fuel" (Nickel-56) to keep it glowing.
The Twist: A standard explosion wasn't enough. The model needed an extra "kick" of energy coming from the collision with the fog.
The Analogy: Think of the explosion as a car engine. The radioactive nickel is the gasoline. But the collision with the fog acts like a turbocharger. It injects extra power into the system, making the car (the supernova) brighter and hotter than it would be on gas alone.

3. The "Double-Decker" Spectra

One of the coolest things the authors discovered involves the shape of the light lines (spectra). When light from a supernova passes through gas, it creates a "P-Cygni" profile: a dip (absorption) on the blue side and a bump (emission) on the red side.

The Discovery: In SN 2023ixf, they saw "kinks" or double dips in these lines.
The Analogy: Imagine looking at a crowd of people running away from you.
- Without the fog: You see one big group running at different speeds. The light shows one smooth dip.
- With the fog (CDS): You see two distinct groups. There's the main crowd (the explosion debris) and a separate, dense group of people (the fog wall) right in front of them.
- The Result: The light shows a "double dip." One dip is from the main crowd, and a sharper, narrower dip appears from the dense wall of fog. This proved that the "fog wall" (CDS) was real and sitting right at the front of the explosion.

4. The Infrared Mystery

The authors also looked at the heat (Infrared) coming from the explosion.

The Problem: Standard models predicted the infrared light would be too dim.
The Solution: The collision with the fog heated up the gas, causing it to glow brightly in infrared, like a radiator heating up a room. The "turbocharger" effect (interaction power) was essential to explain why the supernova was so bright in the heat spectrum.

5. The "Asymmetry" Twist

Finally, the authors wondered: What if the explosion wasn't a perfect sphere? What if the fog was patchy?

The Analogy: Imagine a balloon popping. If the balloon is perfectly round, the air flies out evenly. But if the balloon has a weak spot or is shaped like a peanut, the air shoots out differently depending on which way you look.
The Finding: By simulating a 3D, lopsided explosion, they found that the "kinks" in the light lines could change shape or disappear entirely depending on the angle you viewed them from. This explains why some supernovae look weird or have strange line shapes—it might just be a matter of perspective!

The Bottom Line

This paper tells us that SN 2023ixf was a 15-sun-mass star that exploded with a 1.2 quadrillion trillion joules of energy. It didn't explode in a clean room; it exploded into a thick, messy cloud of its own making.

The collision with this cloud acted like a turbocharger, boosting the brightness and creating a dense shell of gas that we can now see in the light. This study is a masterclass in using computer models to decode the complex physics of a dying star, proving that the "messy" interaction with the surrounding gas is just as important as the explosion itself.

1. Problem and Motivation

SN 2023ixf is a nearby Type II supernova (SN) located at 6.85 Mpc, characterized by early signs of interaction with circumstellar material (CSM). While extensive multi-wavelength observations (X-ray to radio) exist, there is significant debate regarding the progenitor properties (mass, mass-loss history) and the nature of the CSM interaction. Previous studies have yielded disparate results regarding the ejecta mass (ranging from partially stripped to standard Red Supergiant [RSG] progenitors) and explosion energy, often due to degeneracies in light-curve modeling and the neglect of non-local thermodynamic equilibrium (non-LTE) effects.

The specific challenge addressed in this paper is to model the photospheric phase evolution (approx. 20 to 120 days post-explosion) of SN 2023ixf from the Ultraviolet (UV) to the Infrared (IR). The goal is to simultaneously reproduce the spectral energy distribution (SED), line profiles (particularly H $\alpha$ ), and the evolution of the CSM interaction, which standard LTE or radiation-hydrodynamics codes often struggle to capture accurately in this regime.

2. Methodology

The authors employed non-LTE time-dependent radiative-transfer calculations using the code CMFGEN.

Progenitor and Explosion Model:
- The base model is a modified version of the x6p0 model from Hillier & Dessart (2019), representing a 15 $M_\odot$ progenitor that underwent enhanced mass loss during the RSG phase.
- The resulting ejecta mass is 7–8 $M_\odot$ , with a kinetic energy of $1.2 \times 10^{51}$ erg and a $^{56}$ Ni mass of 0.05 $M_\odot$ .
- The chemical composition was remapped from the s15.2 model of Sukhbold et al. (2016) to ensure a robust and detailed abundance structure, while retaining the density and velocity structure of the original x6p0 model.
Interaction and CSM Modeling:
- A Cold Dense Shell (CDS) was introduced at a velocity of 8000 km s $^{-1}$ to represent swept-up CSM.
- Interaction power was injected into the CDS to simulate the ongoing interaction between the ejecta and the CSM. The power was time-dependent, ranging from $\sim 2 \times 10^{42}$ erg s $^{-1}$ at early times ( $\sim 20$ d) down to $\sim 5 \times 10^{40}$ erg s $^{-1}$ at later times ( $\sim 85$ d).
- Clumping: The CDS was modeled with a volume filling factor ( $f_{vol}$ ) of 1% (clumped) to mimic compression and favor recombination, though sensitivity tests were performed up to 100% (smooth).
- Asymmetry: Exploratory 2D axisymmetric calculations were performed using LONG_POL coupled with CMFGEN to test how asymmetries in the CSM or ejecta affect line profiles.
Atomic Physics:
- The models utilized a massive model atom including up to 3000 levels for Fe, Co, and Ni ions, and detailed ionization stages (up to stage IV) for C, N, O, Ne, Mg, Si, and S.
- Five radioactive decay chains ( $^{56}$ Ni, $^{57}$ Ni, $^{52}$ Fe, $^{48}$ Cr, $^{44}$ Ti) were included.
Observational Constraints:
- Models were compared against flux-calibrated spectra and photometry from UV (HST), optical, Near-IR (NIR), and Mid-IR (MIR) observations at epochs ranging from 22.5 d to 115 d.

3. Key Contributions

First Full SED Modeling: This is the first study to model the photospheric phase of SN 2023ixf from the UV to the IR using non-LTE time-dependent radiative transfer, explicitly accounting for CSM interaction power.
CDS Detection via Line Profiles: The study provides a rigorous radiative-transfer explanation for "profile kinks" (double absorptions) observed in optical and IR lines, attributing them to the presence of a CDS rather than high-velocity ejecta or line overlaps.
2D Asymmetry Analysis: The paper demonstrates how axial asymmetry can produce diverse line profile shapes (flat-bottomed troughs, notches at various velocities), explaining the lack of consistent high-velocity features in some lines.
UV Blanketing Analysis: A detailed breakdown of how metal-line blanketing (primarily by Fe II and Ti II) sculpts the UV continuum, showing that interaction power has a modest direct effect on UV flux at these epochs compared to the dominant blanketing effects.

4. Key Results

Progenitor and Explosion Parameters: The model successfully reproduces the duration and brightness of the photospheric phase with a partially stripped 15 $M_\odot$ progenitor (resulting in 7–8 $M_\odot$ ejecta) and a standard explosion energy. This supports studies suggesting a lower ejecta mass than typical Type II SNe.
Role of Interaction Power:
- Interaction power is essential to match the observed UV continuum, optical brightness, and IR flux.
- It boosts the UV flux by a few percent and significantly enhances the IR flux via free-free emission in the ionized CDS.
- It alters line profiles by filling in P-Cygni absorption troughs (particularly in H $\alpha$ ) and creating emission "ledges" on the red side.
CDS Properties:
- The CDS is located at 8000 km s $^{-1}$ and remains at this velocity throughout the photospheric phase.
- There is no evidence for material faster than 8000 km s $^{-1}$ in the absorbing regions at 20–115 days. Apparent high-velocity features are attributed to line overlaps (e.g., Si II in H $\alpha$ ) or asymmetry.
- Double Absorption: As the photosphere recedes below the CDS (after $\sim 50$ days), double absorption features appear in lines like H $\alpha$ , Na I D, and Pa $\gamma$ . These are caused by absorption in the outer CDS and the deeper photosphere.
Infrared Behavior:
- The IR flux is dominated by free-free emission from the CDS.
- Models with interaction power predict lines in pure absorption in the IR, whereas observations show a mix of emission and absorption. This suggests the CDS structure is more complex than the current 1D spherical assumption (e.g., clumping or asymmetry).
Clumping Sensitivity:
- Varying the CDS clumping factor ( $f_{vol}$ ) from 1% to 100% significantly alters the temperature and ionization state.
- Smoother CDS (higher $f_{vol}$ ) leads to higher temperatures, increased ionization, and reduced metal-line blanketing, boosting UV flux but potentially overestimating IR flux.
Asymmetry Effects:
- 2D models show that viewing angle significantly impacts line profiles. An asymmetric CSM can create "notches" or flat-bottomed troughs in P-Cygni profiles, explaining the diversity of observed shapes without requiring high-velocity ejecta.

5. Significance

This work resolves several degeneracies in the interpretation of SN 2023ixf:

Progenitor Mass: It confirms that a lower ejecta mass (7–8 $M_\odot$ ) is required to match the short photospheric phase duration, favoring a progenitor with significant pre-explosion mass loss.
Interaction Mechanism: It establishes that CSM interaction persists throughout the photospheric phase, providing the necessary power to explain the UV and IR excesses and specific line profile morphologies.
Diagnostic Caution: The study warns against interpreting P-Cygni trough features (like high-velocity notches) as definitive proof of high-velocity ejecta or specific chemical abundances (like Helium in Type II SNe), as these can be mimicked by CDS geometry and asymmetry.
Methodological Advance: It demonstrates the necessity of non-LTE, time-dependent radiative transfer coupled with interaction power injection to accurately model interacting SNe, moving beyond the limitations of LTE and simple blackbody approximations.

In conclusion, the paper presents a comprehensive physical model of SN 2023ixf that unifies multi-wavelength observations, confirming the presence of a dense, slow-moving CSM shell and the critical role of ongoing interaction in shaping the supernova's spectral evolution.