⚛️ general relativity

Analyzing GW231109_235456 and understanding its potential implications for population studies, nuclear physics, and multi-messenger astronomy

This paper analyzes the sub-threshold gravitational-wave candidate GW231109_235456 to derive constraints on neutron star mass distributions, equation of state, and kilonova detectability, while projecting that next-generation detectors could significantly improve tidal deformability measurements for similar events.

Original authors: Thibeau Wouters, Anna Puecher, Peter T. H. Pang, Tim Dietrich

Published 2026-09-18

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CC BY 4.0

Original authors: Thibeau Wouters, Anna Puecher, Peter T. H. Pang, Tim Dietrich

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

Deep in the fabric of space and time, massive objects like neutron stars can collide, sending ripples through the universe that we call gravitational waves. These waves are not just ripples; they are a new way of listening to the cosmos, allowing us to hear events that are too dark or too distant to be seen with traditional telescopes. When two neutron stars crash together, they create a violent explosion of energy and matter, often followed by a bright flash of light known as a kilonova. By studying the sound of the collision and the light that follows, scientists can learn about the nature of matter itself, specifically how it behaves under the crushing pressure inside a star. This field of study, which combines the detection of these invisible waves with observations of light and other signals, is called multi-messenger astronomy. It offers a unique chance to understand the fundamental laws that govern the universe, from the smallest particles to the largest structures.

In late 2023, the LIGO-Virgo-KAGRA collaboration, a global network of detectors designed to listen for these cosmic ripples, picked up a faint signal named GW231109. Unlike the loud, clear signals that have been confirmed in the past, this one was quiet, sitting just below the threshold where scientists are usually certain they have found a real event. It was a candidate, a potential whisper from a binary neutron star merger that might have happened about 165 million light-years away. Because the signal was so faint, there was a significant chance it was just noise from the detectors themselves. However, the possibility that it was a real cosmic event was too intriguing to ignore. A team of researchers decided to treat this faint signal as if it were real, using the most advanced tools available to analyze its properties and see what story it might tell about the universe.

The researchers began by running the signal through sophisticated computer models that simulate how gravitational waves are produced when two neutron stars spiral toward each other and collide. They tested the signal against different assumptions about the stars' masses and spins to see which scenario fit best. Their analysis suggested that if this was indeed a real merger, it involved two neutron stars with masses that fit neatly into a pattern scientists have proposed for these objects: a double-peaked distribution where most neutron stars are either slightly lighter or slightly heavier than a specific average. This finding supports the idea that neutron stars are not randomly distributed in mass but follow a specific, predictable pattern. The team also determined that the stars were likely spinning very slowly, which is consistent with what we know about how these stars form and evolve.

One of the most critical questions in this field is what happens to the stars after they crash. Do they bounce back to form a new, larger star, or do they immediately collapse into a black hole? Based on the properties of the signal, the researchers found that the most likely outcome was a prompt collapse. This means that the merged object was so heavy that it could not support itself against its own gravity and fell into a black hole almost instantly, within a fraction of a second. There was a small chance that a temporary, unstable star formed first, but even if it did, it would have lasted only a few milliseconds before collapsing. This rapid collapse has important implications for what we might see in the sky. Because the star collapsed so quickly, it would not have ejected as much material as a slower collapse would, making any resulting explosion of light much dimmer.

The team then calculated what this explosion, or kilonova, would have looked like to telescopes on Earth. They found that because the event was so far away, the light would have been significantly fainter than the famous kilonova observed in 2017. While the signal might have been too dim for some smaller telescopes to catch, they concluded that powerful observatories like the Vera C. Rubin Observatory and the Roman Space Telescope would have had the sensitivity to detect it and track its fading glow over several days. This suggests that even faint, sub-threshold signals can be valuable if we have the right tools to look for their electromagnetic counterparts.

Finally, the researchers looked ahead to the future of gravitational wave astronomy. They simulated what would happen if a similar event were detected by the next generation of detectors, which are planned to be far more sensitive than current ones. Their simulations showed that with these future instruments, scientists could measure the properties of the colliding stars with incredible precision. Specifically, they could determine the "stiffness" of neutron star matter to within about ten percent accuracy. This level of precision would allow scientists to finally pin down the equation of state for dense nuclear matter, a long-standing mystery in physics that describes how matter behaves under extreme pressure. While the current signal was too faint to provide these definitive answers on its own, the study demonstrates that even a single, faint event can guide our understanding of what is possible when the next generation of cosmic listening posts comes online.

Technical Summary: Analyzing GW231109_235456 and its Implications

Problem Statement
The paper addresses the characterization of GW231109_235456 (hereafter GW231109), a sub-threshold gravitational-wave trigger identified during the first part of the fourth observing run (O4a) of the LIGO-Virgo-KAGRA (LVK) collaboration. While initially flagged as a potential binary neutron star (BNS) merger candidate, the signal has a high false alarm rate (FAR) and a low probability of astrophysical origin ( $p_{astro} \approx 0.04$ ). Despite this uncertainty, the authors argue that new BNS observations are critical for population studies and nuclear physics constraints. The primary challenge is to rigorously analyze the source parameters of this low signal-to-noise ratio (SNR) event, assess the robustness of these inferences against prior choices, and determine the event's potential implications for the neutron star equation of state (EOS), merger remnants, and multi-messenger follow-up, while acknowledging the candidate's tentative nature.

Methodology
The authors employ Bayesian inference using the dynesty nested sampler within the bilby framework to sample the posterior distribution of source parameters. Key methodological advancements over previous analyses of this trigger include:

Waveform Models: Utilization of advanced BNS waveform models, specifically IMRPhenomXAS_NRTidalv3 (aligned spins) and IMRPhenomXP_NRTidalv3 (precessing spins), which incorporate the NRTidalv3 tidal phase calibrated for unequal-mass systems with dynamical tides.
Analysis Range: Extension of the analysis duration to 256 seconds and an increase in the maximum frequency to 2048 Hz to enhance sensitivity to tidal effects.
Prior Sensitivity: Systematic testing of various priors to isolate their effects on the posterior:
- Masses: Uniform detector-frame, uniform source-frame, single Gaussian, and double Gaussian population priors.
- Spins: Low-spin ( $a_i < 0.05$ ) and high-spin ( $a_i < 0.4$ ) priors.
- Tidal Deformabilities ( $\Lambda$ ): Uniform sampling, quasi-universal relations (QUR), and sampling constrained by a set of EOSs derived from nuclear theory and pulsar observations.
Downstream Analysis:
- EOS Constraints: Using a metamodel parametrization for the EOS and sampling with Jester (accelerated by GPU) and blackjax (sequential Monte Carlo). Constraints include heavy pulsar mass limits (PSR J1614-2230 and PSR J0740+6620).
- Remnant Fate: Application of machine-learning classifiers (gradient boosting decision trees) trained on $\sim$ 400 numerical relativity simulations to predict the merger outcome (prompt collapse, hyper-massive NS, etc.).
- Kilonova Prediction: Estimation of ejecta masses and lightcurves using the nmma framework and the Bu2019lm model.
- Future Projections: Simulation of a similar source detected by third-generation (3G) detectors (Einstein Telescope and Cosmic Explorer) to project EOS constraints.

Key Results

Source Properties: The component masses of GW231109 are consistent with a BNS system, lying between those of GW170817 and GW190425. The inferred chirp mass is $M_c \approx 1.306 M_\odot$ .
Prior Dependence: Due to the low SNR, the posterior distributions are significantly influenced by prior choices. Broader spin priors result in wider mass posteriors due to mass-spin correlations. However, across all tested priors, the posterior mass distributions show the highest consistency (lowest Jensen-Shannon divergence) with a double Gaussian population model, supporting the hypothesis of a bimodal neutron star mass distribution.
Equation of State (EOS): The candidate GW231109 alone provides negligible constraints on the EOS due to its low SNR. When combined with GW170817 and GW190425, the inclusion of GW231109 has a negligible impact on the inferred radius of a $1.4 M_\odot$ neutron star ( $R_{1.4}$ ) or the TOV mass, as the constraints remain dominated by the high-SNR GW170817 event.
Remnant Fate: Machine learning classifiers predict a prompt collapse to a black hole as the most likely outcome (probabilities ranging from 57% to 83% depending on spin priors). If a hyper-massive neutron star (HMNS) formed, it was likely short-lived, collapsing within 2–5 ms.
Multi-Messenger Implications: The predicted kilonova lightcurves are significantly dimmer than AT2017gfo due to the larger distance ( $\sim$ 165 Mpc). While instruments like ZTF and PRIME might have detected the peak, the LSST and Roman Space Telescope are identified as having sufficient sensitivity to detect such a transient. The paper notes that a reported candidate counterpart (AT2023xqy) peaks roughly 14 days post-merger, making it an unlikely kilonova counterpart given the predicted lightcurve evolution.
Third-Generation Projections: Simulations of a similar source observed by ET and CE networks indicate that 3G detectors could constrain the tidal deformability of a $1.4 M_\odot$ neutron star ( $\Lambda_{1.4}$ ) to approximately 7.5%–12% relative uncertainty, depending on the specific detector design and network configuration.

Significance and Claims
The paper claims to provide a comprehensive, state-of-the-art analysis of a sub-threshold BNS candidate, demonstrating how such events can be utilized to test population models and assess the potential for future multi-messenger astronomy.

Population Studies: The analysis supports the proposed double Gaussian mass distribution for neutron stars, suggesting that even sub-threshold candidates can contribute to population inference when analyzed with appropriate priors.
Methodological Robustness: The work highlights the critical importance of prior selection in low-SNR events, showing that while specific parameter estimates vary, the consistency with specific population models (double Gaussian) remains robust.
Future Outlook: The study underscores the transformative potential of third-generation detectors. It projects that while current low-SNR candidates offer limited EOS constraints, future detectors will be capable of measuring tidal deformability with high precision, thereby significantly advancing our understanding of dense nuclear matter.
Modesty: The authors explicitly state that the astrophysical nature of GW231109 remains uncertain and that the EOS constraints derived from it are weak. They caution that the Bayes factors should not be taken as decisive evidence for a signal and that the downstream analyses (remnant fate, kilonova) strictly assume the trigger is an astrophysical BNS merger.

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