GWTC-4.0: Gravitational-Wave Event Catalog

• GWTC-4.0 is a systematically compiled catalog of transient gravitational-wave events from compact binary coalescences, featuring enhanced selection criteria and extended parameter space.
• The catalog employs multiple detection pipelines, including PyCBC, GstLAL, and BayesWave, to ensure robust signal classification and precise parameter estimation via state-of-the-art waveform models.
• GWTC-4.0 facilitates deep insights into binary evolution and cosmology by providing openly accessible strain data, detailed source properties, and improved statistical tools for population studies.

The Gravitational-Wave Transient Catalog (GWTC-4.0) consists of a systematically compiled record of transient gravitational-wave events, primarily from compact binary coalescences, observed with the LIGO, Virgo, and KAGRA detectors through the first part of the fourth observing run (O4a: 24 May 2023 – 16 January 2024) (Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025, Collaboration et al., 25 Aug 2025). This catalog updates and extends previous releases (GWTC-1/2/3), providing improved coverage in mass, spin, signal-to-noise ratio (SNR), and source classification. GWTC-4.0 is explicitly designed as an accessible and reproducible resource for research on compact object populations, binary evolution, cosmology, and tests of general relativity.

1. Catalog Composition and Data Inclusion

GWTC-4.0 features 128 new candidate events from O4a and a preceding engineering run, adding to the 90 high-confidence events from GWTC-3. Selection criteria require a probability of astrophysical origin $p_\text{astro} \geq 0.5$ and exclusion from vetoes during validation; detailed source property measurements are provided for 86 events with false alarm rate $< 1~\mathrm{yr}^{-1}$ (Collaboration et al., 25 Aug 2025). The catalog comprises binary black hole (BBH), neutron star-black hole (NSBH), and binary neutron star (BNS) mergers, with coverage of extreme source parameters—component masses span $m_1,m_2 \sim 5.8$ to $137~M_\odot$ for BBHs and extend to sub-$3~M_\odot$ for neutron stars.

All underlying strain data, auxiliary channels, calibration uncertainty curves, event parameter files, and electronic catalogues are publicly released at the Gravitational Wave Open Science Center (GWOSC), supporting independent verification, analysis, and population inference (Collaboration et al., 25 Aug 2025).

2. Detection Algorithms, Validation, and Signal Classification

A multi-pipeline approach is employed:

• PyCBC and GstLAL apply matched filtering against large template banks for modeled CBC signals (Collaboration et al., 25 Aug 2025).
• Coherent WaveBurst (cWB) and BayesWave provide unmodeled and minimally modeled burst searches (for poorly modeled or long-duration events) (Macquet et al., 2021, Collaboration et al., 25 Aug 2025).
• False alarm rates are quantified using time-shift background estimation and cross-detector coincidence analysis.
• $p_\text{astro}$ is assigned per event based on the detection pipelines' background models, with a mixture likelihood formalism adopted for population studies (Galaudage et al., 2019).

Candidate events are required to satisfy data-quality criteria, with robust vetoes applied for instrumental or environmental transients. Bayesian glitch subtraction (BayesWave) mitigates non-Gaussian backgrounds for high-purity parameter estimation.

3. Parameter Estimation and Waveform Modeling

Event properties are inferred using state-of-the-art waveform models:

• Aligned-spin and precessing models (IMRPhenomXPHM_SPINTAYLOR, SEOBNRv5PHM, NRSUR7DQ4, IMRPhenomXO4A) are cross-compared for model-systematic evaluation (Collaboration et al., 25 Aug 2025).
• Posterior distributions for component masses, mass ratios ($q = m_2/m_1$), chirp mass ($$\mathcal{M} = \frac{(m_1 m_2)^{3/5}}{(m_1+m_2)^{1/5}}$$), effective inspiral spin ($$\chi_\mathrm{eff} = \frac{m_1 \vec{\chi}_1 + m_2 \vec{\chi}_2}{m_1+m_2}\cdot\hat{L}$$), luminosity distance $D_L$, and redshift $z$ are published per event (Collaboration et al., 25 Aug 2025, Williams, 15 Jan 2024).
• Both minimally modeled (CWB-BBH, BayesWave) and template-based reconstructions are used for waveform consistency checks.

Systematic differences between waveform models, particularly for events with high total mass or large spins (e.g., GW231123_135430, $M\approx236~M_\odot$), are carefully quantified, and consensus estimates reflect both statistical and model-dependent uncertainties.

4. Population Properties and Event Distribution

The GWTC-4.0 population paper identifies persistent over- and under-densities in the BBH mass distribution, notably at $m_1\sim 10~M_\odot$, $20~M_\odot$, and $35~M_\odot$, with a continuum steepening above $35~M_\odot$ (Collaboration et al., 25 Aug 2025). Selected findings:

Feature	Mass/Spin Domain	Implication
Mass overdensity	$m_1 \sim 10, 35~M_\odot$	Deviations from power-law; possible evolutionary effect
Mass ratio peak	$q \sim 0.74^{+0.13}_{-0.13}$ near $m_1\sim10~M_\odot$	Suggests stable mass transfer during binary evolution
Spin magnitude	$<0.57$ for $90\%$ of BHs	Non-extremal, broadly consistent with isolated formation
Spin-orbit alignment	Preferentially aligned	Supports isolated binary evolution, majority of mergers
Negative $\chi_\mathrm{eff}$	$0.24-0.42$ of binaries	Dynamical formation in gas-free environments
Spin-mass-ratio correlation	Effective spin vs. $q$	Evidence for non-trivial dependence

New NSBH candidates (GW230518_125908 and GW230529_181500) provide direct constraints on the lower mass gap (primary $m_1$ possibly $<5~M_\odot$), with secondary masses firmly in the neutron star regime.

Events with SNR above 30 (e.g., GW230814_230901) enable high-fidelity waveform studies, reducing statistical errors and sharpening relativistic and astrophysical tests.

5. Cosmological Measurements and Multi-Messenger Context

The expanded event sample in GWTC-4.0 supports improved standard siren cosmology:

• Luminosity distances from GW measurements combined with host galaxy catalogs (Li et al., 2016), mass modeling, and dark siren techniques permit Hubble constant $H_0$ inference with reduced uncertainty (Collaboration et al., 2021, Mancarella et al., 2022).
• Population-informed updating of $p_\text{astro}$ (probability of astrophysical origin) ensures marginal and sub-threshold events are appropriately weighted, maximizing statistical reliability in population-level cosmological and astrophysical inference (Galaudage et al., 2019).
• The catalog fosters multi-messenger efforts by integrating EM follow-up prioritization schemes (mass and metallicity weighting, Skymap Viewer (Li et al., 2016)) for events with poor GW sky localization.

6. Data Access, Technical Details, and Reproducibility

All data associated with GWTC-4.0—including calibrated LIGO Hanford/Livingston strain, auxiliary channels, analysis products, and catalog event lists—are openly distributed via GWOSC (Collaboration et al., 25 Aug 2025). Calibration is encoded as $h(t) = \Delta L(t)/L$, with $\Delta L(t)$ reconstructed by convolving the error signal with the interferometer response function. Data files are available in hdf5/gwf formats, with metadata on quality segments, calibration envelopes, and event properties.

Analysis workflows mirror those used for GWTC-3, with reproducible pipeline configurations ("Asimov blueprints"), parameter estimation via Bilby/Dynesty, and postprocessing with PESummary (Williams, 15 Jan 2024). Comparative analysis between LVK and community-derived catalogs is conducted using information-theoretic metrics (Jensen-Shannon divergence) to assess posterior consistency.

7. Scientific Impact and Future Directions

GWTC-4.0 represents a maturation in gravitational-wave transient cataloging: doubling the number of confidently identified CBC candidates, extending parameter space coverage, and refining event characterization. The catalog:

• Provides a robust foundation for BBH/BNS/NSBH population studies, rate inference, and binary evolution modeling.
• Enables stringent tests of general relativity and exotic phenomena (e.g., gravitational-wave memory (Hübner et al., 2019), spacetime-symmetry breaking (Haegel et al., 2022)).
• Opens avenues for improved cosmological constraints via standard siren analysis (with ongoing reduction of $H_0$ errors).
• Facilitates ongoing multi-messenger efforts, with catalog-integrated prioritization strategies and increasing synergy with EM observatories.

As future observing runs (later O4 segments, next-generation detectors) commence, GWTC releases will iterate on analysis techniques, candidate selection thresholds, waveform modeling, and population inference—driving refinements in gravitational-wave astronomy and its connections to broader astrophysical and fundamental physics inquiries.