Near-infrared spectroscopy of RS Ophiuchi in 2021: the calm, the storm, and the abatement

This paper presents high-cadence near-infrared spectroscopy of the 2021 eruption of the recurrent nova RS Ophiuchi, characterizing the red giant secondary's pre-eruption state, the high-temperature coronal emission and continuum during the outburst, and potential post-eruption changes in the secondary star.

The Gist

Here is an explanation of the paper "Near-infrared spectroscopy of RS Ophiuchi in 2021: the calm, the storm, and the abatement," translated into everyday language with creative analogies.

The Cast of Characters: A Cosmic Roommate Situation

Imagine a binary star system as a cosmic roommate situation.

• The White Dwarf (The WD): This is the "primary" roommate. It's a dead, super-dense star (the size of Earth but as heavy as the Sun). It's greedy and constantly trying to steal stuff.
• The Red Giant (The RG): This is the "secondary" roommate. It's a massive, bloated, aging star (like a giant, fluffy red pillow). It has a slow, steady "wind" of gas blowing off its surface.

The Plot: The Red Giant is so big that its atmosphere spills over into the White Dwarf's orbit. The White Dwarf gobbles up this gas. Eventually, the pile of gas on the White Dwarf gets so hot and pressurized that it explodes. This is a Recurrent Nova. It's like a pressure cooker that blows its lid off, then refills, and blows again every 15 years or so.

The star system is called RS Ophiuchi (RS Oph). The paper looks at what happened during its 2021 explosion.

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Act 1: The Calm (Before the Storm)

Time: June 2020 (About 14 years after the last explosion).

Before the explosion, the astronomers (the paper's authors) took a "snapshot" of the system using a telescope that sees in Near-Infrared (a type of light our eyes can't see, but which is great for seeing through cosmic dust).

• What they saw: They saw the Red Giant star clearly. It looked like a standard, cool, red giant.
• The "Ghost" Lines: Superimposed on the Red Giant's light were some faint emission lines (glowing streaks of light). The authors figured out these weren't from the Red Giant itself, but from the gas wind it was blowing. Think of it like seeing a streetlamp through a foggy window; the light comes from the lamp, but the fog scatters it. The White Dwarf was shining a harsh light on the Red Giant's wind, making it glow faintly.

Act 2: The Storm (The Explosion)

Time: August 2021.

The pressure cooker blew. The White Dwarf erupted, shooting material out at thousands of kilometers per second.

The "Flash-Ionization" Effect:
When the explosion happened, it sent a massive shockwave of ultraviolet light and heat racing outward. This hit the Red Giant's wind (the "fog" from Act 1) and superheated it instantly.

• The Temperature: The gas heated up to about 8,900 Kelvin (hotter than the surface of the Sun).
• The Visual: The spectrum (the rainbow of light) showed a "bremsstrahlung" continuum. In plain English, this is "braking radiation." Imagine electrons zooming around and suddenly slamming into other particles; they slow down and release energy as light. The whole system was glowing with this hot, glowing gas.

The Coronal Gas (The "Super-Hot" Layer):
While the main gas was at 8,900 K, the astronomers also found evidence of a tiny, incredibly hot layer of gas called "coronal gas."

• The Analogy: If the main gas is a hot summer day (8,900 K), the coronal gas is a nuclear reactor core (1,000,000 K).
• How they knew: They saw specific "coronal lines" (glowing colors that only appear in extreme heat). By day 11, this gas was at 1 million degrees. By day 31, it was still around 800,000 degrees. This is the result of the explosion's shockwave smashing into the wind, creating a cosmic "crash" that heats things to nuclear temperatures.

The Speed Trap:
The astronomers watched the speed of the gas change.

• Day 11: The gas was flying out fast (about 1,900 km/s).
• Day 31: The gas had slowed down significantly (to about 850 km/s).
• Why? Imagine throwing a baseball into a thick wall of water. The ball slows down as it hits the water. The explosion (the ball) hit the Red Giant's wind (the water), creating a shockwave that slowed the debris down.

The "High-Speed" Mystery:
The astronomers tried to catch the gas moving really fast (faster than a minute) to see if there were rapid pulses. They looked at a specific Helium line with high-speed cameras (spectroscopy).

• The Result: Nothing. The gas was steady. No rapid flickering. This is important because it tells us the explosion wasn't pulsing like a strobe light during that phase.

Act 3: The Abatement (The Aftermath)

Time: One year after the explosion.

The storm died down, and the system started to return to normal.

The "New" Red Giant?
When they looked at the Red Giant again a year later, something interesting happened. The chemical "signatures" (molecular bands) of the star looked slightly different.

• The Analogy: It's like looking at a person after a major trauma. They might look the same, but their skin tone or features might have shifted slightly.
• The Finding: The Red Giant seemed to have cooled down slightly or changed its "spectral type" (its classification). The astronomers suspect the explosion might have stripped away some of the outer layers or changed how the star was being irradiated by the White Dwarf, making it look slightly different than it did before the storm.

The Dust Cloud:
In 2006, after a similar explosion, dust formed. In 2021, they saw signs that dust might be forming again, but it took longer. The system is slowly rebuilding its "foggy" atmosphere.

The Big Picture Takeaways

1. It's a Cycle: RS Ophiuchi is a cosmic pressure cooker. It fills up, explodes, and resets.
2. Two Temperatures: During the explosion, there are two distinct "zones": a hot zone (8,900 K) where the wind is heated, and a super-hot zone (1,000,000 K) where the shockwaves collide.
3. The Wind Matters: The explosion doesn't happen in a vacuum; it happens inside the Red Giant's wind. The wind acts like a cushion that slows down the explosion and creates the heat.
4. No Rapid Flickering: The explosion was a steady roar, not a rapid staccato beat, at least during the phase they watched.

Why does this matter?
RS Ophiuchi is a "Type Ia Supernova" candidate. These are the "standard candles" astronomers use to measure the universe. By understanding how these smaller explosions (novae) work, we get better at understanding the massive explosions (supernovae) that happen when these stars finally run out of fuel completely.

In short: The paper is a detailed weather report of a cosmic storm, tracking the temperature, speed, and chemical changes of a star system as it goes from calm, to chaotic, and back to calm again.

Technical Summary

Here is a detailed technical summary of the paper "Near-infrared spectroscopy of RS Ophiuchi in 2021: the calm, the storm, and the abatement."

1. Problem and Scientific Context

RS Ophiuchi (RS Oph) is the best-studied symbiotic recurrent nova (SyRN), consisting of a massive white dwarf (WD, $1.2-1.4 M_\odot$) accreting material from a red giant (RG) companion ($0.68-0.80 M_\odot$). The system undergoes thermonuclear runaways (TNR) on the WD surface approximately every 15–20 years, ejecting material at velocities up to several thousand km/s.

While the 1985 and 2006 eruptions were extensively studied, the 2021 eruption offered a new opportunity to:

• Characterize the pre-eruption state of the circumstellar environment (CSE).
• Monitor the rapid evolution of the shock-heated ejecta and the interaction with the RG wind.
• Investigate the presence of coronal lines and dust formation.
• Determine if the eruption alters the spectral properties of the RG secondary or the accretion disc (AD).
• Search for rapid variability (e.g., quasi-periodic oscillations) in the near-infrared (NIR) during the Super Soft Source (SSS) phase.

2. Methodology

The authors utilized high-cadence, multi-epoch NIR spectroscopy combined with photometry to track the system from $\sim$14 years before the eruption to $\sim$1.7 years after.

• Observations:

  • Instrument: SpeX spectrograph on the NASA Infrared Telescope Facility (IRTF).
  • Wavelength Coverage: 0.7–2.5 $\mu$m (Short-Cross Dispersed, SXD) and 2.7–4.3 $\mu$m (Long-Cross Dispersed, LXD).
  • Epochs:
    • Pre-eruption: June 10, 2020 (Day -428.1).
    • Eruption: August 20, 2021 (Day +11.7) and September 11, 2021 (Day +31.7).
    • Post-eruption: May–July 2022 (Days +281 to +335) and April 2023 (Day +624.1).
  • High Cadence: On Day +31.7, a time-series was obtained with a cadence of $\sim$6–14 seconds to search for rapid line variations.
  • Supporting Data: Swift UVOT photometry (UV) and AAVSO optical photometry were used to construct Spectral Energy Distributions (SEDs) and light curves.

• Analysis Techniques:

  • Spectral Decomposition: Comparison of RS Oph spectra with a standard M2III giant (HD 120052) to isolate emission features.
  • Continuum Modeling: Fitting of bremsstrahlung (free-free) continua and nebular continua to determine electron temperatures ($T_e$) and emitting masses.
  • Line Diagnostics: Analysis of Paschen/Brackett discontinuities ("step heights") to constrain $T_e$.
  • Coronal Temperature Estimation: Using flux ratios of forbidden lines (e.g., [Si VI], [Si X], [S IX]) to derive coronal gas temperatures ($T_{cor}$).
  • Kinematics: Gaussian fitting of line profiles to determine expansion velocities and asymmetries.

3. Key Contributions and Results

A. Pre-Eruption State (The "Calm")

• Spectral Nature: The quiescent spectrum (Day -428.1) is dominated by the RG photosphere (M-type features like TiO and CO bands) with superimposed emission lines.
• Origin of Emission: The emission lines (H recombination, O I, Ca II) were determined to originate primarily from the RG wind, photoionized by the WD and inner accretion disc, rather than the accretion disc itself or silicate dust shells.
  • Calculated electron density ($N_e \sim 10^9$ cm$^{-3}$) and mass estimates ruled out the accretion disc and silicate dust as the primary source of the H recombination lines.
• Continuum: A steep blue rise in the spectrum indicated the presence of the accretion disc, while the H$^-$ continuum was weaker than in field giants, likely due to the harsh radiation environment.

B. The Eruption (The "Storm")

• Thermal Structure:
  • Cool Gas: The NIR continuum and Paschen/Brackett discontinuities indicated a relatively cool, flash-ionized gas at $T_e \approx 8900$ K. This gas is interpreted as the RG wind ionized by the initial UV/X-ray flash of the eruption.
  • Coronal Gas: Strong infrared coronal lines (e.g., [Si X] 1.403 $\mu$m, [Si VII] 1.965 $\mu$m) were detected.
    • Day +11.7: $T_{cor} \approx 10^{6.0}$ K.
    • Day +31.7: $T_{cor} \approx 10^{5.9}$ K.
    • This confirms the presence of shock-heated plasma reaching temperatures of $\sim 10^6$ K.
• Kinematics and Geometry:
  • Day +11.7: H lines showed a main peak ($\sim 680$ km s$^{-1}$) with broad blue/red wings ($\sim -1200$ and $+1900$ km s$^{-1}$). The authors interpret this as a torus-like structure (main peak) with perpendicular outflows (wings).
  • Day +31.7: Significant deceleration was observed. Wing velocities dropped to $\sim -440$ and $+850$ km s$^{-1}$, and line widths narrowed. This aligns with the "Phase II" shock evolution model where ejecta interact with the RG wind ($v \propto t^{-1/3}$).
• Molecular Features:
  • CO Bands: The first overtone CO bands (from the RG) were detected as early as Day +31.7, much earlier than in the 2006 eruption (where they appeared after Day +170). This suggests the RG photosphere was visible sooner or the obscuration was less severe.
  • Dust: No evidence of new dust formation was found in the NIR spectra during the first month, consistent with optical polarization data suggesting dust destruction early on.

C. Post-Eruption and High Cadence (The "Abatement")

• High Cadence Variability: On Day +31.7, during the SSS phase, 40 spectra were taken with $\sim$6s cadence.
  • Result: No rapid ($<1$ min) variability was detected in the He I 1.0833 $\mu$m line.
  • Significance: This contrasts with the 35s Quasi-Periodic Oscillations (QPOs) seen in X-rays, suggesting the NIR emission region does not trace the same rapid physical processes or is too spatially extended to show such modulation.
• Post-Eruption Evolution:
  • Spectral Type Change: By Day +624.1, the TiO bands (specifically $\gamma(0,0)$, $\gamma(0,1)$, and $\epsilon$) became significantly stronger compared to the pre-eruption spectrum. This suggests a change in the effective spectral type of the RG secondary (cooling or reduced irradiation effects) or a change in the viewing geometry (inferior conjunction).
  • P Cygni Profiles: Post-eruption spectra showed P Cygni profiles in He I lines (indicating a wind from the WD) but not in H I lines, suggesting the H emission arises in the permanent circumstellar material while He traces the active WD wind.

4. Significance

• Shock Physics: The paper provides a detailed timeline of the shock deceleration in a SyRN, confirming theoretical models of ejecta-wind interaction ($r \propto t^{2/3}$) and the persistence of coronal temperatures ($\sim 10^6$ K) well into the eruption phase.
• Circumstellar Environment: It clarifies the origin of pre-eruption emission lines, confirming the RG wind as the dominant reservoir for recombination lines, distinct from the accretion disc.
• Progenitor Evolution: The detection of CO bands earlier than in 2006 and the potential shift in the RG's spectral type post-eruption offer new constraints on how recurrent novae affect their donor stars and the stability of the accretion process.
• Multi-wavelength Correlation: The lack of NIR variability despite X-ray QPOs highlights the complexity of the emission regions, suggesting that the NIR continuum and lines are not directly coupled to the innermost accretion flow dynamics on short timescales.

In summary, this study provides a comprehensive "before, during, and after" view of the RS Oph 2021 eruption, utilizing high-cadence NIR spectroscopy to disentangle the contributions of the red giant wind, the accretion disc, and the shock-heated ejecta.