🔭 astrophysics

Neutron star-companion interaction in core collapse supernovae. Population synthesis based on detailed binary evolution models

Using a comprehensive grid of binary evolution models and the SN-ORACLE population synthesis code, this study predicts that interactions between neutron stars and their inflated companions occur in over half of non-disrupted hydrogen-poor core-collapse supernovae, producing periodic light curve modulations lasting up to a decade that explain recent observations like SN2022jli and offer new targets for future detection.

Original authors: Andrea Ercolino, Norbert Langer, Avishay Gal-Yam, Abel Schootemeijer, Caroline Mannes, Harim Jin, Ruggero Valli, Selma de Mink, Luc Dessart

Published 2026-05-21

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Andrea Ercolino, Norbert Langer, Avishay Gal-Yam, Abel Schootemeijer, Caroline Mannes, Harim Jin, Ruggero Valli, Selma de Mink, Luc Dessart

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 Cosmic Dance After a Disaster

Imagine two stars, a massive "big brother" and a smaller "little brother," orbiting each other like dance partners. The big brother runs out of fuel and explodes in a spectacular supernova. Usually, this explosion is so violent that it blows the little brother away, or the two stars drift apart forever.

However, this paper asks a specific question: What happens if the little brother survives the explosion but gets hit by the blast?

The authors used powerful computer simulations to track thousands of these binary star systems. They found that when the explosion hits the companion star, it doesn't just knock it back; it acts like a giant blowtorch, puffing the companion star up like a balloon. Then, the tiny, dense remnant left behind by the explosion (a neutron star) starts orbiting this puffed-up balloon. As the neutron star dips into the balloon's outer layers, it creates a rhythmic "thumping" signal that we can see from Earth.

The Key Players and Events

1. The Explosion (The Supernova)
Think of the supernova as a massive firework going off. The paper focuses on "Type Ic" supernovae (which have lost their hydrogen layers) and "Type Ib" (which have some hydrogen left).

The Hit: When the firework goes off, the debris (ejecta) slams into the companion star. This is called Ejecta-Companion Interaction (ECI).
The Puff: This impact heats the companion star up, causing its outer layers to expand rapidly. The star inflates, becoming much larger and brighter than before, like a balloon being blown up.

2. The Aftermath (The Neutron Star)
The core of the exploded star collapses into a super-dense city-sized ball called a neutron star.

The Kick: When the star explodes, the neutron star often gets a "kick" (like a cue ball in pool), shooting it off in a random direction.
The Orbit: If the kick isn't too strong, the neutron star stays in orbit with the now-inflated companion.

3. The Interaction (CCI)
This is the main discovery of the paper. As the neutron star orbits the puffed-up companion, it occasionally dips into the companion's expanded, low-density atmosphere.

The Analogy: Imagine a tiny, fast boat (the neutron star) skimming across the surface of a giant, soft, expanding cloud (the inflated companion). Every time the boat dips into the cloud, it creates a splash.
The Signal: These "splashes" release energy. Because the orbit is regular, these splashes happen at a regular rhythm. To an astronomer looking from Earth, the supernova's light doesn't just fade away smoothly; it pulsates or undulates (goes up and down) like a heartbeat.

What the Computer Simulations Found

The authors ran a massive simulation called SN-ORACLE (think of it as a cosmic lottery machine) to see how often this happens.

It's Common in "Hydrogen-Poor" Stars: They found that in about 15% to 27% of supernovae where the exploding star has lost its hydrogen (Type Ibc), this "pulsing" interaction happens.
It's Rare in "Hydrogen-Rich" Stars: If the exploding star still has a lot of hydrogen, this interaction is very rare (less than 1%). This is because those stars usually explode in wider orbits where the companion isn't close enough to get hit or stay in orbit.
The Rhythm: The "heartbeat" of these events usually happens every 20 to 50 days. The interaction can last for 6 months to 10 years.

Solving Real Cosmic Mysteries

The paper wasn't just theory; they used their models to explain real events astronomers have actually seen:

SN 2022jli: This supernova was famous for having a perfect 12.4-day "heartbeat" in its light. The authors found specific star models that perfectly match this event. They predict that the companion star is still puffed up and glowing, and we should be able to see it with telescopes like the VLT (Very Large Telescope) for another decade.
SN 2015ap & SN 2022esa: They also found models that explain the rhythmic bumps seen in these other supernovae.

Why This Matters (According to the Paper)

Finding Hidden Stars: Because the companion star gets puffed up and brighter after the explosion, it becomes much easier to spot. The paper predicts we might find these surviving "little brother" stars in the sky for years after the supernova fades.
A New Clue: Previously, finding a surviving companion star was very hard (like finding a needle in a haystack). This paper suggests that instead of looking for the star directly, we can look for the "heartbeat" in the light curve. If we see the rhythmic bumps, we know a neutron star is interacting with a companion.
More Common Than We Thought: The authors conclude that these rhythmic interactions are likely much more common than we realized. We just haven't been looking for the specific "heartbeat" pattern in old data yet.

Summary in One Sentence

This paper uses computer simulations to show that when a massive star explodes, it often puffs up its companion star, causing a newborn neutron star to orbit and "dip" into it, creating a rhythmic heartbeat in the supernova's light that we can detect for years.

Technical Summary: Neutron Star-Companion Interaction in Core Collapse Supernovae

Problem Statement
Recent observations of core-collapse supernovae (CCSNe), particularly the Type Ic event SN2022jli, have revealed periodic modulations in optical flux, H $\alpha$ line velocity, and $\gamma$ -ray emission. These features are hypothesized to arise from the interaction between a newly formed compact object (neutron star, NS) and its non-degenerate stellar companion, following the companion's inflation due to the initial supernova ejecta impact. While the physical mechanism—termed Compact-Object-Companion Interaction (CCI)—has been proposed to explain specific events, the occurrence rate, parameter space, and general observability of such phenomena across the broader population of CCSNe remain unconstrained. Direct detections of surviving companions are rare, necessitating a statistical approach to predict the frequency and properties of these interacting systems.

Methodology
The authors employ the population synthesis code SN-ORACLE, built upon a comprehensive grid of detailed binary stellar evolution models generated with MESA at galactic metallicity. The study integrates three specific physical prescriptions to model the post-explosion evolution:

Natal Kicks: The code assigns natal kicks to newborn NSs using three distinct probability distributions (DM25, K18, and V25) to account for uncertainties in kick magnitudes and directions, including isotropic and polar-aligned scenarios.
Ejecta-Companion Interaction (ECI): Using analytic prescriptions derived from hydrodynamic simulations (Ogata et al. 2021; Hirai et al. 2018), the model calculates the fraction of supernova kinetic energy intercepted by the companion. This energy drives a radial expansion of the companion's envelope to a maximum radius ( $R_{max}$ ) and luminosity ( $L_{max}$ ), lasting for a duration $\tau_{infl}$ .
Compact-Object-Companion Interaction (CCI): The model determines if the newly formed NS penetrates the shock-inflated envelope of the companion. CCI is defined as occurring when the post-supernova periastron distance ( $r_{peri}$ ) satisfies $R_{2} < r_{peri} \leq R_{max}$ , where $R_2$ is the pre-explosion radius. The number of interactions ( $N_{inter}$ ) is calculated based on the orbital period and the inflation timescale.

The study explores a grid of population models varying natal kick prescriptions, explodability criteria (determining NS vs. Black Hole formation), and merger criteria during Roche-lobe overflow (RLOF).

Key Contributions and Results

Occurrence Rates: The study predicts that periodic CCI is a significant phenomenon in hydrogen-poor (H-poor) supernovae (Type Ibc and Ibn). In the reference model, approximately 15.6% of all H-poor supernovae exhibit periodic CCI ( $N_{inter} \geq 2$ ). This rate is highly sensitive to model assumptions, ranging from 3% to 27% across different population models depending on kick distributions and merger criteria. Conversely, CCI is extremely rare ( $\sim 0.01\%$ ) in hydrogen-rich supernovae due to the disruption of wide binaries containing non-degenerate companions.
Orbital and Temporal Properties: Systems undergoing periodic CCI typically possess post-explosion orbital periods ( $P_{post-SN}$ ) peaking between 20 and 50 days, with a broad range of 9.8–226 days. Orbital eccentricities ( $e_{post-SN}$ ) generally peak around 0.4, though a dichotomy exists where low-eccentricity systems arise primarily from progenitors undergoing Case BB RLOF. The interaction duration ( $\tau_{infl}$ ) spans 0.5 to 10 years, with the number of undulations ( $N_{inter}$ ) varying from single events to over 50 cycles.
Companion Characteristics: The ECI-induced inflation increases companion luminosities by factors of 3 to 100, reaching $L_{max} \approx (2.4 - 29.5) \times 10^{38}$ erg s $^{-1}$ ( $\log L/L_{\odot} \approx 4.8 - 5.9$ ). Approximately 7% of H-poor supernovae with periodic CCI leave behind companions exceeding the Humphreys & Davidson limit, appearing as Luminous Blue Variables (LBVs) for short durations. Companion masses typically range from 5 to 30 $M_{\odot}$ .
Reproduction of Observed Events:
- SN2022jli: The models successfully reproduce the 12.4-day modulation period. The best-fitting models predict a companion J-band magnitude of 21–23 for up to $\sim 10$ years, suggesting the companion should be detectable with current instruments like VLT/HAWK-I.
- SN2015ap: The models identify $\sim 70$ binary evolution tracks consistent with the observed 8.4-day period and Type Ib classification.
- SN2022esa: The models produce systems consistent with the $\sim 29-35$ day period, though the lack of H $\alpha$ variability suggests alternative power sources may be at play.

Significance and Conclusions
The paper establishes that periodic CCI is a theoretically robust and potentially common feature of H-poor core-collapse supernovae, occurring in up to a quarter of such events under certain evolutionary assumptions. The authors conclude that the scarcity of identified periodic CCI candidates in current catalogs likely reflects observational limitations rather than physical rarity.

The study provides specific predictions for the post-explosion observability of surviving companions, noting that ECI inflation significantly enhances their brightness, making them detectable for years after the explosion. The authors suggest that systematic re-analysis of archival supernova light curves, combined with high-cadence time-domain surveys (e.g., ZTF, LSST), is necessary to uncover the predicted population of periodic CCI events. These findings offer a pathway to constrain binary evolution parameters, natal kick distributions, and the physics of supernova explosions through the statistical analysis of light curve undulations.