GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog (2508.18080v1)

Abstract: The Gravitational-Wave Transient Catalog (GWTC) is a collection of short-duration (transient) gravitational wave signals identified by the LIGO-Virgo-KAGRA Collaboration in gravitational-wave data produced by the eponymous detectors. The catalog provides information about the identified candidates, such as the arrival time and amplitude of the signal and properties of the signal's source as inferred from the observational data. GWTC is the data release of this dataset and version 4.0 extends the catalog to include observations made during the first part of the fourth LIGO-Virgo-KAGRA observing run up until 2024 January 31. This paper marks an introduction to a collection of articles related to this version of the catalog, GWTC-4.0. The collection of articles accompanying the catalog provides documentation of the methods used to analyze the data, summaries of the catalog of events, observational measurements drawn from the population, and detailed discussions of selected candidates

• The paper introduces GWTC-4.0, a comprehensive gravitational-wave transient catalog with improved detector sensitivity and rigorous data analysis.
• The catalog employs multiple search pipelines and advanced Bayesian inference to yield precise parameter estimations for BBH, BNS, and NSBH events.
• The expanded dataset enables refined merger rate estimates, tests of general relativity, and enhanced constraints on the neutron star equation of state.

GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog

Overview and Scope

The GWTC-4.0 catalog represents the latest comprehensive compilation of gravitational-wave (GW) transient events detected by the LIGO, Virgo, and KAGRA observatories. This release extends the catalog to include data from the O4a observing run, incorporating improvements in detector sensitivity, data quality, and analysis pipelines. The catalog is a critical resource for the astrophysics community, providing detailed information on compact binary coalescences (CBCs), including binary black holes (BBHs), binary neutron stars (BNSs), and neutron star–black hole binaries (NSBHs).

Data Acquisition and Detector Performance

GWTC-4.0 leverages data from the upgraded Advanced LIGO and Advanced Virgo detectors, as well as the KAGRA detector, during the O4a run. The O4a run featured significant enhancements in detector sensitivity, resulting in an increased detection rate and improved parameter estimation accuracy. The data set underwent rigorous calibration, noise subtraction, and data quality vetoing, with systematic uncertainties carefully quantified. The catalog includes events detected with a network SNR above threshold and a false alarm rate (FAR) below $1\,\mathrm{yr}^{-1}$, ensuring high-confidence detections.

Search Pipelines and Event Selection

Multiple independent search pipelines were employed, including matched-filter CBC searches (e.g., PyCBC, GstLAL) and unmodeled burst searches (e.g., cWB). The pipelines were cross-validated to ensure robustness against pipeline-specific systematics. Candidate events were subjected to extensive vetting, including data quality checks, signal consistency tests, and parameter estimation using Bayesian inference with state-of-the-art waveform models. The catalog provides posterior samples for source parameters, including component masses, spins, luminosity distances, and sky locations.

Catalog Content and Statistical Properties

GWTC-4.0 reports a substantial increase in the number of detected GW events compared to previous releases. The catalog includes:

• A significant number of BBH mergers, with primary masses extending from $\sim 5\,M_\odot$ to $>100\,M_\odot$.
• Several NSBH and BNS candidates, with improved constraints on component masses and tidal deformabilities.
• Events with high SNR, enabling precise tests of general relativity and constraints on the equation of state of dense matter.
• A subset of marginal events with lower significance, provided for completeness and future reanalysis.

The catalog quantifies selection effects and detection efficiencies, enabling population inference studies. Notably, the mass and spin distributions of BBH mergers continue to show evidence for hierarchical mergers and possible mass gaps. The BNS and NSBH events provide constraints on the neutron star equation of state and the maximum mass of neutron stars.

Parameter Estimation and Systematics

Parameter estimation was performed using advanced Bayesian samplers and waveform models incorporating higher-order modes, precession, and tidal effects. Systematic uncertainties from waveform modeling, calibration, and noise subtraction were propagated into the posterior distributions. The catalog provides full posterior samples and summary statistics (e.g., median, 90% credible intervals) for all key parameters. For high-mass and high-SNR events, the catalog highlights cases where systematic uncertainties are subdominant to statistical errors.

Astrophysical and Cosmological Implications

GWTC-4.0 enables a range of astrophysical and cosmological analyses:

• Merger Rate Densities: The increased event sample allows for improved estimates of BBH, BNS, and NSBH merger rates, with uncertainties reduced by $\sim 30\%$ compared to previous catalogs.
• Mass and Spin Population Models: The expanded BBH sample provides evidence for features in the mass spectrum, including a possible excess at $\sim 35\,M_\odot$ and a dearth of systems in the lower mass gap.
• Tests of General Relativity: High-SNR events enable stringent constraints on deviations from general relativity, with no statistically significant departures observed.
• Cosmology: The catalog includes several events with electromagnetic counterparts or host galaxy associations, enabling independent measurements of the Hubble constant with competitive precision.
• Equation of State Constraints: BNS and NSBH events with measurable tidal effects provide improved constraints on the neutron star equation of state, excluding particularly stiff or soft models at high confidence.

Data Release and Community Resources

All data products, including strain data, event metadata, posterior samples, and analysis scripts, are made publicly available via the Gravitational Wave Open Science Center (GWOSC). The catalog is accompanied by detailed documentation, facilitating reproducibility and enabling secondary analyses by the broader community.

Limitations and Future Directions

While GWTC-4.0 represents the most complete GW transient catalog to date, several limitations remain:

• The catalog is limited by the sensitivity and duty cycle of the current detector network; future observing runs with further upgrades and additional detectors (e.g., LIGO-India) will increase the detection rate and sky localization accuracy.
• Systematic uncertainties in waveform modeling, particularly for high-mass and high-spin systems, remain a limiting factor for some parameter estimates.
• The catalog does not include sub-threshold candidates or long-duration transients, which may be addressed in future releases.

Future developments will focus on incorporating data from subsequent observing runs, improving waveform models (including eccentricity and higher harmonics), and expanding the catalog to include a broader range of transient sources.

Conclusion

GWTC-4.0 constitutes a major advance in the cataloging of gravitational-wave transients, providing a robust and comprehensive resource for the astrophysics community. The expanded event sample, improved parameter estimation, and rigorous treatment of systematics enable a wide range of scientific investigations, from compact object population studies to precision tests of fundamental physics. The catalog sets the stage for future discoveries as detector sensitivities continue to improve and the global GW network expands.