Matter-Binary Catalog Status

Calling a compact-binary event BNS or NSBH is an inference, not a label read from strain. Classification can use component-mass likelihoods, tidal information, an assumed maximum neutron-star mass, population priors, and electromagnetic counterparts. These information sources have different strength across events. As of 2026-08-18, a small set of systems robustly establishes neutron-star mergers, while several compact-object classifications remain prior or model sensitive; GWTC-5.0 adds no new confidently matter-containing O4b event.

Helpful background. What a Binary-Black-Hole Signal Measures explains detector/source-frame parameters and classification. Equation-of-State Inference shows why matter evidence is not equivalent to a mass threshold.

Classification is a model comparison

For class $C$—BNS, NSBH, or BBH—the posterior probability is schematically

$$p(C\mid d) \propto p(C) \int_{\Theta_C}d\theta\, p(d\mid\theta,C)\pi(\theta\mid C).$$

The class domain $\Theta_C$, prior odds $p(C)$, mass and spin distributions, EOS/tidal model, and waveform all matter. A hard boundary at a chosen maximum neutron-star mass can make classification appear more certain than a model that marginalizes over that maximum.

Candidate significance is separate. $p_{\rm astro}$ compares an astrophysical signal process with terrestrial noise under a search model. Conditional class probabilities such as $p_{\rm BNS}$, $p_{\rm NSBH}$, and $p_{\rm BBH}$ divide the signal population. They should not be conflated or imported into a different population analysis without their assumptions.

Four kinds of matter evidence

Mass support. A low secondary mass can favor a neutron star, but low-mass black holes or massive neutron stars may overlap near uncertain boundaries.

Tidal response. Nonzero deformability is more direct matter evidence, but current event likelihoods may be consistent with both small positive and zero tides.

Electromagnetic counterpart. A kilonova and short gamma-ray burst associated with a gravitational-wave event strongly support matter outside the remnant. Association, emission modeling, and the distinction between BNS and disruptive NSBH still require inference.

Population context. Known neutron-star and black-hole mass distributions change class odds, especially for weak tidal data. This is useful but makes the classification population conditional.

Absence of a counterpart is not evidence for a BBH unless follow-up coverage and emission-model detectability make the non-detection informative.

Event status at the cutoff

Event or release	Durable detector-level reading	Classification boundary
GW170817	Long low-mass inspiral, tidal constraints, and a securely associated multimessenger transient	Strong BNS interpretation; detailed EOS/remnant claims remain model dependent
GW190425	Low-mass compact-binary inspiral consistent with a heavy BNS	No confirmed counterpart; mass, spin, and population priors carry more classification weight
GW200105 and GW200115	Compact binaries with secondaries strongly consistent with neutron-star masses	Tides were not resolved; NSBH probabilities depend on maximum-mass and population assumptions
GW230529	Asymmetric system with one neutron-star-like object and a companion in the lower compact-object mass interval	Gravitational waves alone do not definitively determine whether each object is NS or BH
GWTC-5.0 O4b addition	New confidently cataloged O4b systems are consistent with BBHs	No new O4b matter event changes the earlier matter sample

GW170817’s discovery and multimessenger association provide the strongest classification chain LVK 2017, pp. 2–8. GW190425’s official analysis explains its heavy-BNS interpretation and lack of counterpart LVK 2020, §§2–5. GW200105/115 establish NSBH systems under explicitly varied astrophysical assumptions LVK 2021, pp. 2–8.

GW230529 is deliberately described more cautiously: the official analysis finds its most probable interpretation to include a neutron star and a low-mass black hole, while stating that the gravitational-wave data alone do not definitively determine both object types LVK 2024, pp. 2–7. The current cumulative O4b status comes from GWTC-5.0 LVK 2026, observations §§1 and 3.

Robustness checks

For each event, report classification under:

• low and broad spin priors;
• waveform models with and without tides, precession, and relevant higher modes;
• alternative maximum-neutron-star-mass and EOS priors;
• population-informed and minimally informative mass priors;
• calibration, PSD, and glitch variants;
• counterpart association and selection models when applicable.

A class is robust when its posterior odds remain decisive across credible alternatives and are supported by a discriminating observable. A classification driven mainly by excluding parameter space through one prior should be stated as such.

Catalog-level matter rates require search-specific selection for BNS, NSBH, and BBH populations. Different signal durations and masses yield different efficiencies, and marginal candidates require a contamination mixture. One event should not be assumed representative of the entire NSBH mass distribution.

What could change the conclusion

This status must be refreshed after a new catalog, revised event likelihood, counterpart association, improved maximum-mass constraint, or waveform reanalysis that changes class odds. The page reports evidence through its cutoff; current alert classifications belong in dated services until final data products exist.

Exercises

1. Astrophysical versus BNS probability. A trigger has $p_{\rm astro}=0.9$ and, conditional on being astrophysical, $p_{\rm BNS}=0.6$. What is the joint probability of “astrophysical and BNS” under that same model?

Solution

It is $0.9\times0.6=0.54$. The remaining probability includes astrophysical non-BNS classes and terrestrial noise. This multiplication is meaningful only because the second probability was explicitly conditional on the first model’s astrophysical hypothesis.

2. A mass-only NS label. Why can changing the maximum neutron-star mass prior alter an NSBH probability even when the event likelihood is unchanged?

Solution

The prior changes which component-mass region belongs to the neutron-star class and how much prior weight it receives. The evidence integral over each class therefore changes. Without resolved tides or a counterpart, the data may not independently fix that boundary.

References

• LIGO Scientific Collaboration and Virgo Collaboration. “GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral.” Physical Review Letters 119 (2017): 161101. DOI. Open PDF.
• LIGO Scientific Collaboration and Virgo Collaboration. “GW190425: Observation of a Compact Binary Coalescence with Total Mass $\sim3.4M_\odot$.” Astrophysical Journal Letters 892 (2020): L3. DOI. Open PDF.
• LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration. “Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences.” Astrophysical Journal Letters 915 (2021): L5. DOI. Open PDF.
• LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration. “Observation of Gravitational Waves from the Coalescence of a $2.5$–$4.5M_\odot$ Compact Object and a Neutron Star.” Astrophysical Journal Letters 970 (2024): L34. DOI. Open PDF.
• LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration. “GWTC-5.0: Observations from the Second Part of the Fourth LIGO-Virgo-KAGRA Observing Run and Updates to the Gravitational-Wave Transient Catalog.” arXiv:2605.27225 (2026). Open PDF.