GWTC-3: Gravitational-Wave Transient Catalog

• GWTC-3 is a comprehensive catalog of gravitational-wave detections from 90 validated compact binary merger candidates, including black hole and neutron star events.
• It employs multiple independent search pipelines to determine robust astrophysical probabilities and derive precise source properties like masses, spins, and redshift evolution.
• The catalog underpins rigorous tests of general relativity, constraints on cosmological parameters, and advances in multi-messenger astrophysics.

The Gravitational Wave Transient Catalog 3 (GWTC-3) is a comprehensive catalog of gravitational-wave (GW) detections compiled by the LIGO and Virgo Collaborations, representing signals observed during the first three observing runs of the advanced GW detector network. It encompasses a statistically validated list of 90 compact binary coalescence candidates (with $p_\mathrm{astro} > 0.5$), spanning binary black hole (BBH) mergers, neutron star–black hole (NSBH) mergers, and candidate binary neutron star (BNS) systems. GWTC-3 enables detailed population inference, stringent tests of general relativity (GR), constraints on cosmological parameters and modified gravity, and supports the broader program of multi-messenger astrophysics.

1. Catalog Composition, Detection Methods, and Significance Metrics

GWTC-3 includes candidates identified during the second half of the third observing run (O3b; Nov 2019–Mar 2020) and consolidates all prior events to a total of 90, processed using multiple independent search pipelines: PyCBC, GstLAL, MBTA, and Coherent WaveBurst (CWB) (Collaboration et al., 2021). Candidate events require a probability of astrophysical origin $p_\mathrm{astro} > 0.5$, calculated as: $$p_\mathrm{astro} = \frac{\Lambda_\mathrm{signal}}{\Lambda_\mathrm{signal} + \Lambda_\mathrm{noise}}$$ where $\Lambda_\mathrm{signal}$ and $\Lambda_\mathrm{noise}$ are the expected rates of astrophysical signals and noise triggers, respectively. This approach ensures robust background estimation and cross-pipeline validation.

The 35 new O3b candidates consist predominantly of BBH mergers, with the catalog also featuring the first confident NSBH detections. Precise classification of the components is sometimes limited by the lack of direct matter effects in the gravitational signal, making distinctions such as “neutron star vs black hole” model-dependent in borderline mass systems.

2. Source Properties: Mass, Spin, and Redshift Distributions

Analysis of the GWTC-3 population yields detailed distributions for source parameters (Collaboration et al., 2021). The BBH mass spectrum exhibits well-defined features, with chirp mass ($\mathcal{M}$) peaks at $8.3^{+0.3}_{-0.5}\,M_\odot$ and $27.9^{+1.9}_{-1.8}\,M_\odot$, and a primary mass distribution that decays sharply above $60\,M_\odot$. The BNS mass distribution is broad and flat from $1.2^{+0.1}_{-0.2}$ to $2.0^{+0.3}_{-0.3}\,M_\odot$, and significant constraints are placed on the location of the “mass gap” between neutron stars and black holes.

The BBH merger rate is tightly constrained, with strong evidence for a redshift evolution of the form: $$\mathcal{R}_{BBH}(z) \propto (1 + z)^{\kappa}, \qquad \kappa = 2.9^{+1.7}_{-1.8}$$ for $z \lesssim 1$. This mirrors the global star-formation history and delay time distribution of binaries.

Spin measurements, augmented by hierarchical mixture modeling, show at least some BBH mergers have spin vectors closely aligned (field formation) and others exhibit evidence for extreme tilt angles $>90^\circ$ (dynamical formation), with a Bayes factor $\sim$10 in favor of a sub-population with significant spin misalignment (Tong et al., 2022). Overall, at most $89\%$ of BBHs are dynamically assembled at $99\%$ credibility; the remainder are consistent with isolated (field) formation.

3. Tests of General Relativity and Exotic Physics

GWTC-3 data enable an unprecedented battery of “null” and alternative-theory tests (Collaboration et al., 2021):

• No statistically significant deviation from GR across all tested domains: parameterized post-Newtonian (PN) deformation coefficients, consistency between inspiral and ringdown, non-GR polarization content, residuals, and post-merger echoes.
• Constraints on graviton mass: $m_g \leq 1.27 \times 10^{-23} \mathrm{eV}/c^2$.
• Spin-induced quadrupole moments of BBHs are consistent with the Kerr hypothesis ($Q = -\kappa \chi^2 M^3$ with $\kappa \approx 1$).
• No compelling evidence for Lorentz- or CPT-violating effects in GW propagation: stringent bounds from effective field theory formulations (SME) on both isotropic and anisotropic coefficients, with $|k_{(V)00}^{(5)}| \leq 3.19 \times 10^{-15}$ m and anisotropic parameters $\sim 10^{-13}$ m (Haegel et al., 2022).
• No evidence for amplitude birefringence or parity violation: the birefringence attenuation parameter $\kappa = -0.019^{+0.038}_{-0.029}$ Gpc$^{-1}$ at $100$ Hz, corresponding to a parity-violation energy scale $M_{PV} \gtrsim 6.8 \times 10^{-21}$ GeV (Ng et al., 2023).
• Population joint likelihoods further tighten bounds on deformation coefficients and propagation modifications.

4. Cosmological Applications: Expansion History and Modified Gravity

GWTC-3’s high-redshift reach (up to $z \sim 1$) allows gravitational-wave “standard siren” cosmology without electromagnetic counterparts (Collaboration et al., 2021, Mancarella et al., 2022, Chen et al., 16 May 2024):

• Redshift is inferred statistically by matching the observed distribution of redshifted source-frame masses to a universal mass model, or by marginalizing over host galaxies in galaxy catalogs (GLADE+).
• Measured Hubble constant: $H_0 = 68^{+8}_{-6}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ (with dark siren constraints dominated by the optical counterpart of GW170817), and significantly improved precision versus GWTC-1 and GWTC-2.
• Modified GW propagation parameter $\Xi_0 = 1.2^{+0.7}_{-0.7}$ (flat prior), with $\Xi_0=1$ recoverable in GR, yielding the tightest bounds yet on extra friction effects.
• The spectral siren method uses the redshift evolution of the BBH mass spectrum and luminosity distance to constrain the GW friction parameter $\nu$, finding $\nu=0.5^{+3.5}_{-2.6}$ at 90% credibility and improving on previous constraints by an order of magnitude (Chen et al., 16 May 2024).
• Observed propagation is entirely compatible with general relativity; deviations implied by running Planck mass or exotic friction are strongly constrained.

5. Methodological Advances: Waveform Models, Surrogate Analyses, and Workflow

The parameter estimation for GWTC-3 events leverages several waveform families (IMRPhenomXPHM, SEOBNRv4PHM), with recent work demonstrating that full precessing numerical relativity surrogate models (NRSur7dq4/Remnant) offer improved fidelity for mass ratios $q > 1/6$ and total masses $>60 M_\odot$ (Islam et al., 2023). NRSur7dq4 captures all $\ell \leq 4$ modes and precession effects, and produces statistically significant differences in masses, spins, and extrinsic parameters compared to semi-analytical models, especially for highly precessing or short-inspiral events (e.g., GW190521, GW191109).

Bayesian model comparison and Jensen–Shannon divergence metrics are used to quantify differences between posteriors from alternative models and between independently derived events/candidates in community catalogs (Williams, 15 Jan 2024).

Automated, reproducible analysis workflows (e.g., using YAML “blueprints” in the Asimov system) enable scalable data handling, reproducible PE, and fast catalogue extensions, crucial for incorporating community events and for posterior consistency analyses.

6. Multi-messenger Constraints and Non-GW Counterpart Searches

GWTC-3 is tightly integrated into multi-messenger strategies, with systematic electromagnetic and neutrino counterpart searches:

• X-ray: Swift-XRT followed up 18 GW triggers from O3, setting upper limits of $\sim3.6 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ (0.3–10 keV) and providing luminosity bounds of $\sim10^{44}$ erg s$^{-1}$ for BBH events at $\sim$474 Mpc (Page et al., 2020).
• Gamma-ray: Swift-BAT analysis via the maximum-likelihood NITRATES pipeline yields upper limits in 15–350 keV for all GWTC-3 events, and computes GW–BAT joint false alarm rates. No significant prompt EM counterparts were found, and the measured limits offer stringent constraints on exotic models of EM emission from BBH mergers (Raman et al., 13 Jul 2024).
• Neutrinos: Borexino sets the most stringent bounds to date on low-energy (0.5–5 MeV) neutrino and antineutrino fluences from GW events; upper limits at $10^9$–$10^{15}$ cm$^{-2}$ per event are reported, constraining both standard and exotic emission pathways (Collaboration et al., 2023).

7. Legacy and Future Directions

GWTC-3 constitutes a foundational reference for gravitational wave population astrophysics, fundamental physics, and multi-messenger astronomy. The catalog enables:

• Continuous refinement of merger rates, mass and spin distributions, and the evolution of compact binaries across cosmic time.
• More stringent and robust tests of GR in the strong-field regime and constraints on beyond-GR models, including Lorentz/CPT violation, parity violation, massive graviton scenarios, and deformations of the Kerr metric (Santos et al., 26 Mar 2024).
• The integration of community events and the development of automated, scalable analysis infrastructure for the next-generation catalogs.
• Preparing the field for “multiband” gravitational wave astronomy as future detectors, both space-based (e.g., LISA) and advanced ground-based, allow for joint coverage and tighter cross-validation of source parameter estimates (2206.12439).

GWTC-3 and its associated analyses represent an unprecedented advance in our empirical understanding of the compact binary population, the behavior of gravity in extreme conditions, and the landscape for future discoveries in gravitational-wave and multi-messenger astrophysics.