GWTC-4.0: Gravitational-Wave Event Catalog

• GWTC-4.0 is a comprehensive catalog of gravitational-wave transients from compact binary coalescences, featuring rigorous Bayesian analyses and enhanced event validation.
• The catalog introduces 128 new events with extreme SNR detections and detailed parameter estimations, advancing population studies and tests of general relativity.
• Advanced signal processing, noise mitigation, and hierarchical Bayesian models underpin the catalog’s robust statistical inferences and astrophysical insights.

The Gravitational-Wave Transient Catalog 4.0 (GWTC-4.0) is the latest official release by the LIGO–Virgo–KAGRA Collaboration of gravitational-wave (GW) transient events detected primarily from compact binary coalescences. GWTC-4.0 incorporates a significant expansion in event count, methodology, and data quality relative to earlier releases, providing a comprehensive reference for GW astrophysics, foundational tests of general relativity, and multi-messenger astronomy. The catalog consists of both new and previously reported mergers, with enhanced data from the first segment of the O4 observing run (O4a, May 2023 to January 2024), and is underpinned by rigorous Bayesian analysis pipelines, systematic data-quality assessments, and population-level inferences. The catalog is accessible via the Gravitational Wave Open Science Center and is supported by a collection of methodological and population-statistics companion publications.

1. Catalog Expansion, Event Types, and Notable Detections

GWTC-4.0 adds 128 new compact binary coalescence (CBC) candidates from the O4a observing interval, raising the total number of unpublished, non-vetoed, high-probability ($p_{\rm astro} \geq 0.5$) detections to 218, a more than twofold increase over GWTC-3.0. The catalog encompasses:

• Binary Black Hole (BBH) mergers: The dominant source class, showing a source-frame primary mass range of $5.79\,M_\odot$ (GW230627_015337) up to $137\,M_\odot$ (GW231123_135430; component masses exceeding the pair-instability mass gap).
• Neutron Star–Black Hole (NSBH) binaries: Two new candidates, GW230518_125908 (mass ratio $q\simeq0.18^{+0.04}_{-0.03}$, low effective inspiral spin) and GW230529_181500 (primary possibly in the lower mass gap).
• Events with extreme network SNR: For the first time, GW signals with network SNR > 30 (GW230814_230901, GW231226_01520) have been observed, allowing highly precise waveform and parameter measurements.

This expansion yields improved statistical power for rate inference, population analyses, and strong-field gravity tests.

2. Data Processing, Signal Identification, and Quality Control

Candidate identification utilizes a suite of matched-filter pipelines (using waveform banks that span the spin, mass, and orientation parameter space) and unmodeled burst searches (e.g., coherent WaveBurst). Events are required to meet stringent thresholds for false alarm rate (FAR < 1 yr⁻¹ for detailed parameter estimates). Matched filtering computes optimal SNR via the noise-weighted inner product $$(d|h) = 4\,{\rm Re}\int_{f_{\rm min}}^{f_{\rm max}} \frac{d(f) h^*(f)}{S_n(f)} df,$$ where $S_n(f)$ is the detector noise power spectral density.

Extensive data-quality monitoring is performed with both auxiliary channel-based vetoes and in-band glitch identification. For events affected by instrumental noise artifacts (e.g., transient glitches), advanced noise subtraction (using linear and Bayesian approaches such as BayesWave) or raised frequency cutoffs are applied to recover the astrophysical signal. Calibration uncertainties are carefully marginalized in likelihood evaluations.

3. Parameter Estimation and Systematic Uncertainties

Source parameter inference is performed using several independent Bayesian samplers (notably LALInference, Bilby, and related codes) and an expanded suite of waveform models. These models now include:

• Higher-order multipole moments,
• Spin precession,
• Tidal effects (for BNS/NSBH),
• Surrogate and reduced-order models for high-mass, high-spin, and asymmetric mergers.

In cases of pronounced waveform-model dependency (e.g., for GW231123_135430, the most massive BBH identified to date), parameter estimates are presented for each model, and samples are often aggregated to provide joint results. Multimodal posterior distributions (seen in select events for chirp mass and mass ratio) highlight ongoing challenges in interpreting complex signals in the presence of spin-precession or high mass ratios.

4. Population Properties: Mass, Spin, and Formation Channel Inference

Population-level analysis leverages hierarchical Bayesian modeling, accounts for selection effects, and uses up-to-date detection efficiencies:

• Primary mass distribution for BBH systems is best characterized by a broken power law, $p(m_1)\propto m_1^{-\alpha}$ with a “break” near $36\,M_\odot$ and a steepening at higher masses ($\alpha_2\simeq4.5$).
• Low-mass and “gap” features are observed, including pronounced densities at $\sim10\,M_\odot$ and $\sim35\,M_\odot$, with a third possible feature near $20\,M_\odot$.
• Mass ratio distribution peaks at $q=0.74^{+0.13}_{-0.13}$ for primaries near $10\,M_\odot$, potentially indicative of stable mass transfer in isolated binary evolution.
• Spin constraints: 90% of black holes have dimensionless spin $\chi<0.57$. Black hole spins are preferentially aligned with the orbital angular momentum, supporting isolated field evolution for a substantial fraction. Conversely, $24$–$42$\% of binaries show negative effective inspiral spins, suggesting dynamical formation in gas-poor environments.
• Correlation analysis reveals a trend between effective inspiral spin ($\chi_{\rm eff}$) and mass ratio, though it is not fully disentangled whether variation is mode- or width-driven.

Collectively, GWTC-4.0 supports a mixed formation channel scenario, with both isolated evolution (aligned, moderate-spin systems) and dynamical processes (randomly aligned, possibly lower mass-ratio and $\chi_{\rm eff}<0$ systems) contributing to the observed population (Collaboration et al., 25 Aug 2025).

5. Methodological Advances and Data Release

GWTC-4.0 leverages major methodological improvements:

• Simultaneous operation of multiple search and inference pipelines,
• Greater automation and standardization in event validation,
• Standardized data products and parameter estimates cross-published in machine-readable formats via the Gravitational Wave Open Science Center (GWOSC),
• Systematic error budgets include calibration, glitch mitigation, and model/systematics uncertainties,
• Data on sub-threshold candidates (i.e., those not meeting the highest significance standards) are released to facilitate auxiliary searches (e.g., for electromagnetic or neutrino coincidences or for future population analysis refinement) (Nitz et al., 2021, Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025).

The formalism for handling marginal events, evolving priors, and appropriately weighting ambiguous detections with $p_{\rm astro}<1$ is directly incorporated (Galaudage et al., 2019), and catalog variance effects in population and strong-field gravity testing are now rigorously quantified using bootstrapping approaches (Pacilio et al., 2023).

6. Astrophysical and Fundamental Physics Implications

The expanded catalog enables:

• More precise BBH, BNS, and NSBH merger rate estimates,
• Improved constraints on the black hole mass gap, low-mass and high-mass end behaviors,
• Enhanced tests of general relativity (e.g., via inspiral–merger–ringdown consistency, ringdown spectroscopy, parameterized deviations),
• Broader and deeper population studies, quantifying the relative contributions of different formation channels,
• Optimized strategies for multimessenger follow-up given improved localization and mass-, spin-, and metallicity-informed targeting,
• Tightened bounds on GW memory, gravitational-wave dispersion, and parity violation, in synergy with ongoing companion analyses.

7. Future Prospects and Catalog Evolution

GWTC-4.0 sets a new standard for gravitational-wave event catalogs, but the field anticipates further expansion with continued observing runs (e.g., O4b/O4c), the full operation of KAGRA and ongoing sensitivity upgrades. The statistical power of GWTC-4.0, combined with refined technical methodologies, positions the catalog as a foundation for future population synthesis studies, multi-messenger astronomy, and increasingly precise tests of gravity—in preparation for the era of continuous GW event detection and cataloging.

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Table: Key GWTC-4.0 Metrics

Metric	Value/Range	Notes
$N_{\rm events}(p_{\rm astro}\geq0.5)$	218	Doubled vs. GWTC-3.0
Primary BBH mass range	5.79–137 $M_\odot$	$137\,M_\odot$ (GW231123_135430)
Max. network SNR	$\sim$42.1 (GW230814_061920)	Highest fidelity waveform to date
Effective spin, $\chi$	90% have $\chi<0.57$	Non-extremal, skewed toward alignment
Merger rate (BBH, $z=0$)	$16.5_{-6.2}^{+10.4}$ Gpc$^{-3}$ yr$^{-1}$	Population modeling, (Nitz et al., 2021)
Notable methods	Hierarchical Bayesian modeling, glitch mitigation, model-marginalized inference

GWTC-4.0 thus represents the most comprehensive, systematically vetted, and statistically rigorous catalog of gravitational-wave transients to date (Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025).