Unveiling the nature of SN 2022jli: The first double-peaked stripped-envelope supernova showing periodic undulations and dust emission at late times

This paper presents optical and infrared observations of the peculiar stripped-envelope supernova SN 2022jli, which exhibits a double-peaked light curve with periodic 12.5-day undulations and late-time dust emission, suggesting a scenario where a neutron star formed in a binary system powers the secondary maximum through accretion while potentially linking magnetar activity to broader features in stripped-envelope supernovae.

The Gist

Imagine a star as a massive, glowing engine that has been running for millions of years. Eventually, it runs out of fuel, collapses under its own weight, and explodes in a spectacular event called a supernova. Usually, these explosions are like a single, massive firework: they get bright, reach a peak, and then slowly fade away.

But SN 2022jli was not your average firework. It was a cosmic mystery that behaved like a double-decker cake with a hidden, rhythmic heartbeat.

Here is the story of this peculiar explosion, broken down into simple concepts:

1. The Double-Peak Surprise

Most supernovae have one "peak" of brightness. Think of it like a single drumbeat. SN 2022jli, however, had two distinct peaks separated by about 50 days.

• The First Peak: This was a standard explosion. The star blew up, shone brightly, and started to fade. Scientists calculated that this first burst was powered by the radioactive decay of nickel (like a tiny, glowing battery inside the debris).
• The Second Peak: Just as the star was supposed to be fading into the darkness, it suddenly flared up again! It reached the same brightness as the first explosion. This was the first time astronomers saw a "stripped-envelope" supernova (a star that had lost its outer layers of hydrogen and helium) do this.

2. The Cosmic Heartbeat

After that second explosion, things got weird. Instead of fading smoothly, the light began to pulse.

• Imagine a lighthouse beam that doesn't just spin, but flickers on and off with perfect rhythm.
• SN 2022jli pulsed every 12.5 days. It got brighter, then dimmer, then brighter again, like a cosmic heartbeat. This rhythm was so precise that it suggested something mechanical or orbital was happening, rather than just a random explosion.

3. The Mystery of the "Ghost" Companion

So, what caused the second peak and the rhythmic pulsing? The paper suggests a dramatic scenario involving a binary system (two stars orbiting each other).

• The Setup: The exploding star was likely part of a pair. When the main star exploded, it left behind a dense core (a neutron star). Its partner star was still there.
• The Accretion: As the neutron star and its partner orbited each other, they got close every 12.5 days. At these closest points (called periastron), the neutron star "ate" or sucked material from its partner.
• The Analogy: Imagine a child on a swing. Every time the swing comes close to a parent, the parent gives them a push. In this case, the neutron star gets a "push" of energy every time it swoops close to its partner, causing the light to flare up (the second peak) and creating the rhythmic undulations.

4. The Dusty Aftermath

As the explosion cooled down over the next year, something else happened.

• The CO Signal: Astronomers detected carbon monoxide (CO) gas forming in the debris. This is like finding the first bricks being laid to build a house in the middle of a storm.
• The Dust Cloud: About 200 days after the explosion, the supernova started glowing brightly in infrared light (heat). This was caused by dust forming in the cooling debris.
• The Analogy: Think of a campfire. When the fire is hot, you see bright flames. As it cools, the ash and soot start to glow red hot. SN 2022jli created a massive cloud of cosmic soot (dust) that glowed for a long time. This is important because dust is the building block of new stars and planets.

5. The "Magnetar" Twist

While the "eating partner" theory explains the rhythm, the paper also suggests a second ingredient: a Magnetar.

• A magnetar is a neutron star with a magnetic field so strong it could tear a credit card apart from a million miles away.
• The scientists propose that the explosion didn't just leave a normal neutron star; it left a super-charged magnetar. This magnetar might have been spinning incredibly fast, injecting extra energy into the explosion to create that second, massive peak of light.

Why Does This Matter?

This event is a "Rosetta Stone" for astronomers.

1. It proves binary stars can explode in weird ways: It shows that when stars live in pairs, their deaths can be much more complex and energetic than if they were alone.
2. It tracks dust creation: We got a front-row seat to watch dust being born in real-time, which helps us understand how the universe gets the ingredients to make new solar systems.
3. It connects the dots: It suggests that the most energetic, "super-bright" supernovae we see in the universe might all be powered by these same magnetars and binary interactions, just on a larger scale.

In short: SN 2022jli was a star that didn't just die; it had a dramatic, rhythmic farewell party, powered by a hungry neutron star and a super-magnet, while simultaneously building a cloud of cosmic dust that will one day become part of a new world.

Technical Summary

Here is a detailed technical summary of the paper "Unveiling the nature of SN 2022jli: The first double-peaked stripped-envelope supernova showing periodic undulations and dust emission at late times."

1. Problem and Scientific Context

Stripped-envelope (SE) supernovae (SNe IIb, Ib, Ic) typically exhibit single-peaked light curves powered by the radioactive decay of $^{56}\text{Ni}$. While double-peaked light curves have been observed in rare SE SNe (e.g., SN 2005bf, PTF11mnb), the mechanisms driving the secondary maximum and subsequent variability remain debated. Proposed scenarios include:

• Radioactive Nickel Distribution: A double-peaked distribution of $^{56}\text{Ni}$.
• Compact Object Power: Energy injection from a newly formed magnetar or accretion onto a compact object (neutron star/black hole).
• Circumstellar Interaction (CSM): Interaction between ejecta and dense circumstellar material.
• Binary Interaction: Accretion from a companion star or collision with a companion.

SN 2022jli, discovered in the nearby galaxy NGC 157, presents a unique case with a double-peaked light curve, unprecedented periodic undulations ($P \sim 12.5$ days) following the second peak, and significant late-time infrared (IR) excess. The paper aims to characterize these phenomena to determine the progenitor system and the power source of the explosion.

2. Methodology

The authors conducted a comprehensive multi-wavelength analysis of SN 2022jli from maximum light to approximately +800 days.

• Observations:
  • Optical Photometry: Data from ZTF, ATLAS, ASAS-SN, Gaia, and ground-based telescopes (LDSS-3/Clay, EFOSC2/NTT, Goodman/SOAR).
  • Near-Infrared (NIR) Photometry: Obtained with Flamingos-2 on Gemini-South (J, H, $K_s$ bands) and NEOWISE (W1, W2 bands).
  • Spectroscopy: Optical spectra from LDSS-3, Goodman, and GMOS-S; NIR spectra from Flamingos-2 and Triple-Spec (SOAR).
• Data Analysis:
  • Light Curve Modeling: Fitted polynomial functions to determine peak epochs and decline rates. Used Lomb-Scargle periodograms to analyze periodicity in residuals.
  • Bolometric Reconstruction: Constructed pseudo-bolometric light curves by integrating optical spectra and estimating NIR contributions using templates from SN 2013ge.
  • Physical Parameter Estimation: Applied Arnett's (1982) analytical model to the first peak to derive ejecta mass ($M_{ej}$) and $^{56}\text{Ni}$ mass ($M_{Ni}$).
  • Spectral Analysis: Measured expansion velocities via Fe II lines, identified spectral features (He I, H$\alpha$, CO overtone), and modeled nebular phase lines ([O I], [Ca II]).
  • Dust Modeling: Fitted NIR/MIR photometry with optically thin dust emission models (graphite and silicate) to derive dust temperature ($T_d$) and mass ($M_d$).

3. Key Results

A. Light Curve and Periodicity

• Double-Peaked Structure: SN 2022jli exhibited two maxima separated by $\sim$50 days, both reaching a peak luminosity of $\sim 3 \times 10^{42}$ erg s$^{-1}$.
• Periodic Undulations: Following the second maximum, the light curve displayed periodic undulations with a period of $P \approx 12.5$ days. These were most prominent in bluer bands (g, c) and correlated with color variations (bluer at flux maxima).
• Late-Time Drop: After $\sim$+350 days, the optical luminosity dropped significantly, falling below the expected decline from $^{56}\text{Ni}$ decay, suggesting the cessation of the secondary power source.

B. Physical Parameters (First Maximum)

• The first peak is consistent with a standard SN Ic event.
• Ejecta Mass ($M_{ej}$): $\sim 1.5 \pm 0.4 M_{\odot}$.
• $^{56}\text{Ni}$ Mass ($M_{Ni}$): $\sim 0.12 \pm 0.01 M_{\odot}$.
• Expansion Velocity: $\sim 8500$ km s$^{-1}$ at maximum light, declining linearly at a rate of $\sim 13.5$ km s$^{-1}$ day$^{-1}$.

C. Spectral Evolution

• Classification: Confirmed as a SN Ic, though weak He I features were detected, suggesting incomplete envelope stripping.
• Hydrogen Signatures: A double-peaked H$\alpha$ and Pa$\beta$ feature emerged after the second maximum. The peak positions of these lines shifted periodically, tracking the light curve undulations. This is interpreted as accretion of hydrogen-rich material from a binary companion.
• Nebular Phase: Late-time spectra ($>+400$ days) showed strong [O I] $\lambda\lambda$6300, 6364 and [Ca II] $\lambda\lambda$7291, 7323 lines. The [O I]/[Ca II] flux ratio of $\sim 2.2$ confirms the SE SN nature.

D. Molecular and Dust Emission

• CO Overtone: The first CO overtone emission (2.3–2.5 $\mu$m) was detected from +190 to +224 days but disappeared by +400 days.
• Dust Emission: A significant IR excess appeared around +200 days, dominating the $K_s$ band by +380 days.
  • Dust Temperature: Ranged from 900 K to 650 K.
  • Dust Mass: Estimated at $2 - 16 \times 10^{-4} M_{\odot}$ (depending on composition: graphite vs. forsterite).
  • Origin: Likely newly formed dust in the ejecta or a strong IR echo from pre-existing dust.

E. Power Source Modeling

• Magnetar Scenario: The second maximum and the overall light curve can be modeled by a hybrid scenario: radioactive decay of $0.12 M_{\odot}$of$^{56}\text{Ni}$plus energy injection from a magnetar ($B \sim 8.5 \times 10^{14}$G,$P \sim 48$ ms).
• Accretion Scenario: The periodic undulations and shifting H$\alpha$ lines are best explained by super-Eddington accretion of material from a companion star onto the central compact object (neutron star) at periastron passages in a binary system.

4. Significance and Conclusions

This paper presents the first SE SN exhibiting both a double-peaked light curve and strict periodic undulations. The key contributions and implications are:

1. Hybrid Power Mechanism: SN 2022jli supports a model where a magnetar powers the secondary maximum (providing the extra luminosity and blue colors), while binary accretion drives the periodic undulations and hydrogen line variability. This bridges the gap between magnetar-powered SLSNe and binary interaction models.
2. Progenitor System: The detection of periodic hydrogen features strongly implies a binary progenitor system where the SN ejecta interacts with a non-degenerate companion star, challenging the single-star Wolf-Rayet progenitor model for some SE SNe.
3. Dust and Molecule Formation: The detection of CO and the rapid formation of a significant dust mass ($\sim 10^{-4} M_{\odot}$) in a SE SN provides crucial insights into the chemical evolution of SN ejecta and the contribution of core-collapse SNe to the cosmic dust budget.
4. Late-Time Behavior: The sudden drop in optical luminosity after the magnetar/accretion phase ceases highlights the transient nature of these non-radioactive power sources in SE SNe.

In summary, SN 2022jli serves as a critical laboratory for understanding the diversity of SE SN explosions, the role of binary interactions in stripping envelopes, and the complex interplay between magnetars, accretion, and dust formation in the aftermath of stellar death.