GWTC-4.0: Gravitational-Wave Catalog

• GWTC-4.0 is a comprehensive gravitational-wave transient catalog detailing compact binary coalescences observed during the O4a run.
• It employs advanced data analysis methods, including matched filtering and Bayesian parameter estimation, to robustly validate gravitational-wave signals.
• The catalog expands event statistics, refining measurements of masses, spins, and merger rates to enhance our understanding of binary evolution and astrophysical populations.

GWTC-4.0 is the fourth major release of the Gravitational-Wave Transient Catalog compiled by the LIGO-Virgo-KAGRA (LVK) Collaboration (Collaboration et al., 25 Aug 2025). It comprises an extensive catalogued set of gravitational-wave (GW) transient detections, chiefly sourced from compact binary coalescences (CBCs)—including binary black holes (BBHs), binary neutron stars (BNSs), and neutron star–black hole (NSBH) mergers—observed during the early stage of the fourth observing run (O4a, spanning May 24, 2023 to January 16, 2024). The GWTC-4.0 data release incorporates 128 new CBC candidates detected in this time window, bringing the total number of catalogued candidates with astrophysical probability $p_{\rm astro} \geq 0.5$ to 218. The catalog is notable for both its methodological advances in data analysis and its enriched source population statistical studies, setting the benchmark for contemporary GW transient cataloging.

1. Scope and Structure of the Catalog

GWTC-4.0 aggregates short-duration GW signals characterized by their arrival time, amplitude, source localization, and physical properties inferred from Bayesian parameter estimation (Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025). Events are identified via multiple search pipelines (including PyCBC, GstLAL, and coherent WaveBurst), each employing matched filtering against banks of template waveforms grounded in post-Newtonian and effective-one-body (EOB) approximations. Candidate selection uses a combination of detection statistics (signal-to-noise ratio, SNR), false-alarm rate (FAR), and estimates of astrophysical probability ($p_{\rm astro}$). High-confidence events (FAR $< 1\,\mathrm{yr}^{-1}$) are subject to detailed parameter-estimation studies across multiple waveform families.

The catalog documents precise arrival times, three-dimensional sky localization, strain amplitudes, and frequency-domain measurements. Table organization includes event name, $p_{\rm astro}$, FAR, component masses, source classification, spin parameters, and distance estimates.

Event Name	$p_{\rm astro}$	Class (BBH/BNS/NSBH)
GW230518_125908	$\geq 0.5$	NSBH
GW231123_135430	$\geq 0.5$	BBH
...

The inclusion criteria and validation protocols yield an expanded set of catalogued CBCs, notably increasing the event rate and population statistics compared to GWTC-3.0 (Collaboration et al., 25 Aug 2025).

2. Advanced Data Analysis Methodology

GWTC-4.0 employs tightly integrated methods for GW transient identification and characterization (Collaboration et al., 25 Aug 2025). Signal modeling is achieved via sophisticated waveform approximants (IMRPhenomXPHM_SPINTAYLOR, SEOBNRv5PHM, NRSUR7DQ4), spanning the full inspiral–merger–ringdown cycle. Matched filtering, defined in the frequency domain by

$$(a|b) = 4 \operatorname{Re} \int_{f_\mathrm{min}}^{f_\mathrm{max}} \frac{\tilde{a}^*(f) \tilde{b}(f)}{S_n(f)}\,df,$$

where $S_n(f)$ is the detector noise PSD, yields the detection SNR

$\rho = \frac{(d|h)}{\sqrt{(h|h)}}.$

Transients are classified using cross-detector coincidence tests and non-Gaussian noise artifact (glitch) mitigation, employing the BAYESWAVE algorithm. Parameter estimation leverages Bayesian inference with likelihood

$$\mathcal{L}(d|\theta) \propto \exp\left[-\frac{1}{2} (d-h(\theta)|d-h(\theta))\right],$$

and posterior

$$p(\theta|d) \propto \pi(\theta)\mathcal{L}(d|\theta),$$

where $\theta$ are the physical source parameters. Multiple waveform models are used to quantify systematic uncertainty; for high-SNR events, multimodal posterior distributions are observed, indicating model-dependent inference (Collaboration et al., 25 Aug 2025).

Catalog management integrates outcome from all search pipelines and applies strict waveform-model consistency tests, ensuring only validated astrophysical events are included.

3. Source Property Measurements

Source classification is primarily into BBH, NSBH, and (possible) BNS systems. Parameter estimation derives source-frame component masses and chirp mass ($\mathcal{M}$):

$$\mathcal{M} = \frac{(m_1 m_2)^{3/5}}{(m_1 + m_2)^{1/5}},$$

where $m_1$, $m_2$ are source-frame masses. BBH median masses now range from $5.79\,M_\odot$ (GW230627_015337) to $137\,M_\odot$ (GW231123_135430), with GW231123_135430 interpreted as the most massive binary observed.

Effective inspiral spin ($\chi_{\rm eff}$) and precession spin ($\chi_p$) are measured for each candidate:

$$\chi_{\rm eff} = \frac{m_1 \chi_{1,\perp} + m_2 \chi_{2,\perp}}{m_1 + m_2}, \quad \chi_p = \max\left[ \chi_{1,\perp}, \frac{q(4q + 3)}{4 + 3q}\chi_{2,\perp} \right],$$

with $q = m_2/m_1 \leq 1$. Two BBH candidates (GW231028_153006 and GW231118_005626) feature $\chi_{\rm eff} \approx 0.4$; several events display significant aligned or anti-aligned spin, with one (GW231123_135430) inferred to have primary spin magnitude $|\chi_1| \sim 0.9$.

NSBH candidates (GW230518_125908, GW230529_181500) have secondary components with $m_2 < 3\,M_\odot$, supporting classification as black hole–neutron star systems. Localization accuracy varies; some events are constrained to sky areas $\sim 110\,\mathrm{deg}^2$, with distance estimates ranging from $\sim 0.2\,\mathrm{Gpc}$ to $\sim 7\,\mathrm{Gpc}$.

Systematic differences among waveform families yield multimodal and broadened uncertainties for key parameters, necessitating careful model selection and statistical cross-validation.

4. Population Properties and Astrophysical Inferences

Analysis of 158 mergers from the cumulative GWTC-4.0 yields high-fidelity statistical population models (Collaboration et al., 25 Aug 2025). The BBH mass distribution displays multiple over- and under-densities with primary-mass features at $10\,M_\odot$, $20\,M_\odot$, $35\,M_\odot$, and a steepening above $35\,M_\odot$. The broken power-law model parameters for a subdominant mode are:

• $\alpha_1 \simeq 1.7$ (lower-mass slope)
• $\alpha_2 \simeq 4.4$ (upper-mass slope)
• $m_\text{break} \simeq 36\,M_\odot$ (break mass)
• peaks near $m_{\text{pp},1} \simeq 9.8\,M_\odot$ and $m_{\text{pp},2} \simeq 28$–$29\,M_\odot$

The BBH mass-ratio distribution peaks at $q=0.74^{+0.13}_{-0.13}$, with nearly equal-mass binaries present in one population submode. The dimensionless spin parameter distribution is sharply non-extremal: $90\%$ of black holes have $\chi < 0.57$, with most spins preferentially aligned with orbits, consistent with formation in isolation. However, $24$–$42\%$ of binaries possess negative effective inspiral spins—an indicator that dynamical formation in non-gaseous environments is substantial.

Redshift evolution is parameterized, yielding a subdominant-mode median merger rate near $16\,\text{Gpc}^{-3}\,\text{yr}^{-1}$, with dominant mode rates around $29\,\text{Gpc}^{-3}\,\text{yr}^{-1}$. Joint mass-spin inference provides astrophysical constraints on progenitor channels, including possible signatures of stable mass transfer and/or cluster dynamical interactions.

5. Systematics, Data Quality, and Event Validation

Robustness of GWTC-4.0 results is upheld through advanced glitch mitigation and data quality strategies (Collaboration et al., 25 Aug 2025). Several events required time–frequency excision of noise transients, implemented via raising the low-frequency cutoff and/or applying the BAYESWAVE algorithm for minimally-modeled waveform reconstruction. Overlap studies between on-source reconstructions and off-source injections confirm the absence of missing waveform content.

Ambiguous candidates—such as GW230630_070659 (affected by scattered light) and GW230824_135331—are presented with detailed spectrograms, guiding cautious interpretation on marginal event classification. Procedures for sky localization and three-dimensional volume containment are quantitatively documented.

6. Implications for Fundamental Physics and Future Research

GWTC-4.0 offers a dataset for rigorous tests of general relativity in the strong-field regime and propels further statistical paper of CBC populations (Collaboration et al., 25 Aug 2025). High-SNR events enable inspiral–merger–ringdown consistency checks and stringent bounds on deviations from predicted waveforms. The catalog facilitates population inference, cosmological standard siren analyses, and lensing investigations.

Expanded event counts and refined analysis pipelines (including the Asimov blueprint-driven workflow in community catalogs (Williams, 15 Jan 2024)) enhance reproducibility and scalability in future releases. As the GW detector network matures (e.g. with complete KAGRA operation), sky-localization, mass–spin inference precision, and merger rate estimates are expected to advance.

A plausible implication is that increasing statistics and methodological rigor may reveal subtle features in the mass, spin, or formation-channel distributions, and may further constrain or reveal deviations from general relativistic predictions.

7. Catalog Evolution and Comparative Context

GWTC-4.0 builds upon the analytical framework, computational architecture, and open-data practices established in previous releases (Ghosh, 2022, Williams, 15 Jan 2024). Compared to GWTC-3.0, GWTC-4.0 doubles the event sample, introduces novel search methods, and applies improved calibration, noise mitigation, and multi-model inference procedures.

Developments such as the YAML workflow management system (Asimov), direct sampling from cosmological redshift priors, and updated parameter estimation libraries (Bilby v2.1.1) foster consistent and efficient catalog generation (Williams, 15 Jan 2024).

Methodological consistency and cross-catalog comparison metrics (e.g., Jensen–Shannon divergence) are used for benchmarking and validation, enabling integration of independently derived event parameters and facilitating joint population studies.

The catalog’s evolution reflects a broader commitment to methodological transparency, open science, and statistical robustness, providing a foundation for the next generation of gravitational-wave astrophysics.