Hunting Gravitational Waves from Supernovae
When a massive star exhausts its nuclear fuel, it collapses catastrophically, rebounding in one of nature's most violent events: a core-collapse supernova. Theory predicts these events should produce gravitational waves. Finding them would revolutionize our understanding of stellar death.
Our recent work, published in arXiv:2410.16565, presents targeted searches for gravitational waves from two nearby supernovae: SN 2023ixf and SN 2023zaw.
Why Supernovae Should Produce Gravitational Waves
Core-collapse supernovae involve extreme physics:
The Collapse
When a massive star's core runs out of fuel:
- Iron core collapse: ~1.4 solar masses collapses in less than a second
- Neutron star formation: Protons and electrons combine, forming neutrons
- Bounce: The collapse halts, rebounding violently
- Shock wave: Energy propagates outward, eventually exploding the star
This process is inherently asymmetric, with:
- Rotating core: Angular momentum creates non-spherical collapse
- Convection: Turbulent material flow
- Neutrino-driven dynamics: Neutrinos depositing energy asymmetrically
Any mass asymmetrically accelerated produces gravitational waves.
Expected Signal Characteristics
Supernova gravitational waves differ from binary mergers:
- Frequency: ~100-2000 Hz (LIGO's sensitive band)
- Duration: Seconds, not milliseconds
- Waveform: Complex, not clean chirps
- Amplitude: Much weaker than binary mergers
The challenge: extracting weak, complicated signals from detector noise.
SN 2023ixf: The Pinwheel Galaxy Supernova
On May 19, 2023, amateur astronomer Koichi Itagaki discovered a supernova in M101, the Pinwheel Galaxy—just 21 million light-years away.
Why This Was Special
SN 2023ixf was:
- Nearby: One of the closest supernovae in decades
- Well-observed: Detected early, tracked extensively across wavelengths
- Type II-P: Core-collapse from red supergiant, well-understood progenitor
- During O4: LIGO-Virgo-KAGRA were operating with good sensitivity
Perfect conditions for a gravitational wave search.
Our Analysis
We searched LIGO Hanford and LIGO Livingston data from May 19, 2023, when the supernova was discovered.
Methods Used:
Unmodeled Burst Search
- No assumptions about waveform morphology
- Looks for excess power in time-frequency space
- Sensitive to unexpected signals
Model-Based Search
- Templates from supernova simulations
- Different explosion mechanisms
- Varying rotation rates and asymmetries
Stochastic Search
- Looking for random gravitational wave background
- Sensitive to multiple overlapping signals
The Result
No detection.
But "no detection" is scientifically valuable.
What We Learned
Our null detection allowed us to set upper limits on:
Energy Radiated Less than 0.01 solar masses converted to gravitational wave energy—consistent with theoretical predictions.
Rotation Rate Constraining how fast the proto-neutron star was spinning at birth.
Asymmetry Limits on how non-spherical the collapse was.
These constraints inform supernova simulations, helping theorists refine models.
SN 2023zaw: The Ultraviolet Surprise
SN 2023zaw, discovered in April 2023, was different:
- Type Ibn: Interaction with helium-rich circumstellar material
- Bright ultraviolet: Unusual emission characteristics
- Well-positioned: Good localization for multi-messenger follow-up
Different Physics, Different Expectations
Type Ibn supernovae involve interaction between the explosion and material previously expelled by the star. This interaction could enhance gravitational wave emission through:
- Enhanced asymmetry: Clumpy circumstellar material
- Shock dynamics: Complex multi-shock structure
- Energy conversion: Kinetic energy to gravitational waves
Our search employed similar methods to SN 2023ixf but with templates adapted for the different explosion mechanism.
Results
Again, no detection. Again, valuable constraints.
The Challenge of Supernova Gravitational Waves
Why haven't we detected supernova gravitational waves yet?
Distance Problem
Gravitational wave amplitude decreases with distance: A ∝ 1/d
Even SN 2023ixf at 21 million light-years is relatively distant on gravitational wave scales. For comparison:
- Binary black holes: We detect mergers billions of light-years away
- Supernovae: Need events within ~1 million light-years for detection
Nearby core-collapse supernovae are rare—perhaps 2-3 per century in our galaxy and its immediate neighborhood.
Signal Complexity
Supernova signals are:
- Non-standard waveforms: Not the clean chirps from binary mergers
- Model-dependent: Different explosion mechanisms produce different signals
- Short duration: Limited time to accumulate signal-to-noise
This makes detection significantly harder than binary mergers.
Detector Sensitivity
Current LIGO-Virgo-KAGRA sensitivity is marginal for all but the most optimistic supernova models at realistic distances.
Next-generation detectors like Einstein Telescope and Cosmic Explorer will detect supernova gravitational waves throughout our galaxy and nearby galaxies—dozens to hundreds per year.
Multi-Messenger Observations
What made our searches powerful was coordination with electromagnetic observations:
Precise Timing
Knowing exactly when the supernova occurred let us search specific time windows, improving sensitivity.
Distance Information
Electromagnetic observations provided accurate distances, constraining expected signal amplitudes.
Explosion Mechanism
Spectroscopy revealed the type of supernova, informing our choice of waveform models.
This multi-messenger approach exemplifies modern astronomy.
The Importance of Null Results
In science, negative results are valuable:
Constraining Theory
Our upper limits exclude certain theoretical models. Simulations predicting high gravitational wave emission from these events are ruled out.
Improving Methods
Each search teaches us about our algorithms, our sensitivities, our systematic effects. We learn for future events.
Building Confidence
When we do detect supernova gravitational waves, the community will trust the result because we've demonstrated our methods work and our thresholds are robust.
The Galactic Supernova Dream
The supernova gravitational wave community dreams of one event: a galactic core-collapse supernova.
SN 1987A
The last nearby supernova was SN 1987A in the Large Magellanic Cloud (160,000 light-years away) in February 1987.
LIGO didn't exist yet. What we'd give for that event today.
Betelgeuse Watch
Betelgeuse, a red supergiant in Orion, could go supernova anytime in the next 100,000 years. At just 650 light-years away, we would detect its gravitational waves with stunning clarity.
The LIGO-Virgo-KAGRA collaboration maintains real-time supernova alerts, ready to respond within minutes to a galactic event.
Looking Forward
Next-Generation Detectors
Einstein Telescope and Cosmic Explorer will detect supernova gravitational waves in:
- Milky Way: Every event with exquisite detail
- Local Group: M31, LMC, SMC events easily detected
- Nearby galaxies: Hundreds of detectable supernovae per year
Neutrino Coincidence
Supernovae emit ~99% of their energy in neutrinos. The next galactic supernova will be detected by:
- IceCube: Antarctic neutrino observatory
- Super-Kamiokande: Japanese water Cherenkov detector
- JUNO: Once operational
Combining gravitational wave and neutrino observations will reveal the explosion mechanism in unprecedented detail.
Continuous Improvement
Each search refines our techniques:
- Better noise characterization
- More sophisticated algorithms
- Expanded waveform catalogs
- Faster follow-up procedures
When the next nearby supernova occurs, we'll be ready.
Personal Reflections
Working on these searches connected me with the broader LIGO collaboration's supernova group—brilliant scientists across the globe united by shared curiosity.
The intensity of a supernova search is unique. When an event occurs, we drop everything, analyzing data round-the-clock to produce results while the astronomical community watches expectantly.
Even without a detection, contributing to these searches feels meaningful. We're pushing the boundaries of what's observable, preparing for the discoveries to come.
Conclusion
Our searches for gravitational waves from SN 2023ixf and SN 2023zaw didn't find signals. But they advanced the field, constrained theory, and honed our methods.
The next nearby supernova might come tomorrow or in a century. When it does, the LIGO-Virgo-KAGRA collaboration will be listening, ready to capture nature's most violent explosions through the lens of spacetime itself.
Based on arXiv:2410.16565: Search for Gravitational Waves from Core-Collapse Supernovae