GWTC-4.0: Updating the Gravitational-Wave Transient Catalog with Observations from the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run (2508.18082v1)

Abstract: Version 4.0 of the Gravitational-Wave Transient Catalog (GWTC-4.0) adds new candidates detected by the LIGO, Virgo, and KAGRA observatories through the first part of the fourth observing run (O4a: 2023 May 24 15:00:00 to 2024 January 16 16:00:00 UTC) and a preceding engineering run. In this new data, we find 128 new compact binary coalescence candidates that are identified by at least one of our search algorithms with a probability of astrophysical origin $p_{\rm astro} \geq 0.5$ and that are not vetoed during event validation. We also provide detailed source property measurements for 86 of these that have a false alarm rate $< 1 \rm{yr}^{-1}$. Based on the inferred component masses, these new candidates are consistent with signals from binary black holes and neutron star-black hole binaries (GW230518_125908 and GW230529_181500). Median inferred component masses of binary black holes in the catalog now range from $5.79\,M_\odot$ (GW230627_015337) to $137\,M_\odot$ (GW231123_135430), while GW231123_135430 was probably produced by the most massive binary observed in the catalog. For the first time we have discovered binary black hole signals with network signal-to-noise ratio exceeding 30, GW230814_230901 and GW231226_01520, enabling high-fidelity studies of the waveforms and astrophysical properties of these systems. Combined with the 90 candidates included in GWTC-3.0, the catalog now contains 218 candidates with $p_{\rm astro} \geq 0.5$ and not otherwise vetoed, doubling the size of the catalog and further opening our view of the gravitational-wave Universe.

• The paper presents an expanded gravitational-wave transient catalog doubling event detections with 218 CBC candidates using advanced search pipelines.
• It employs improved calibration, noise subtraction, and multiple waveform models to robustly estimate source properties and mitigate systematic uncertainties.
• The catalog reveals diverse binary systems—including high-SNR BBH, NSBH candidates, and extreme mass events—paving the way for future astrophysical studies.

GWTC-4.0: Expansion of the Gravitational-Wave Transient Catalog with O4a Observations

Catalog Expansion and Event Identification

GWTC-4.0 represents a substantial update to the LIGO-Virgo-KAGRA (LVK) Gravitational-Wave Transient Catalog, incorporating data from the first part of the fourth observing run (O4a) and a preceding engineering run. The catalog now includes 218 compact binary coalescence (CBC) candidates with a probability of astrophysical origin $p_\mathrm{astro} \geq 0.5$ and not vetoed during event validation, more than doubling the previous census. Of these, 128 new candidates were identified in O4a, and detailed source property measurements are provided for 86 events with false alarm rate (FAR) $< 1\,\mathrm{yr}^{-1}$.

The search methodology leverages four pipelines—CWB-BBH, GSTLAL, MBTA, and PyCBC—each with distinct algorithmic approaches and sensitivity profiles. Offline analyses, benefiting from improved calibration and noise subtraction, yield higher sensitivity and more robust candidate lists compared to online, low-latency searches. Pipeline consistency studies reveal that high-SNR events are reliably recovered across all pipelines, while lower-SNR and single-detector events exhibit greater variability in significance and astrophysical probability estimates.

Source Property Inference and Population Extremes

Bayesian parameter estimation is performed for high-purity candidates, utilizing multiple waveform models (IMRPhenomXPHM_SpinTaylor, SEOBNRv5PHM, NRSUR7DQ4, IMRPhenomXO4A) to mitigate systematic uncertainties. The catalog encompasses a broad range of source properties:

• Masses: BBH candidates span total masses from $14.2^{+0.8}_{-0.4}\,M_\odot$ (GW230627_015337) to $236^{+29}_{-48}\,M_\odot$ (GW231123_135430), with the latter likely representing the most massive binary observed to date. Several events have remnant masses $M_f \geq 100\,M_\odot$, qualifying as intermediate-mass black holes.
• Mass Ratio and Spin: Notable events include GW231028_153006 ($M=152^{+29}_{-14}\,M_\odot$, $q=0.63^{+0.33}_{-0.35}$, $\chi_\mathrm{eff}=0.4^{+0.2}_{-0.2}$) and GW231118_005626 ($M=30.9^{+5.3}_{-3.6}\,M_\odot$, $q=0.55^{+0.37}_{-0.22}$, $\chi_\mathrm{eff}=0.4^{+0.1}_{-0.1}$, $\chi_1=0.65^{+0.28}_{-0.38}$), both exhibiting significant spin and mass asymmetry.
• NSBH Candidates: GW230518_125908 and GW230529_181500 are consistent with neutron star-black hole binaries, with secondary masses $m_2 < 3\,M_\odot$ and primary masses in the putative lower mass gap ($3$–$5\,M_\odot$).
• Signal-to-Noise Ratio: GW230814_230901 and GW231226_101520 are the first BBH signals with network SNR $>30$, with GW230814_230901 reaching SNR $42.1$.

Multimodal posteriors are observed in several candidates, particularly in mass and spin parameters, often correlated with waveform model systematics or low-SNR data. Systematic differences between waveform models are subdominant to statistical uncertainties for most events, but are pronounced for high-mass and high-spin systems.

Search Sensitivity and Subthreshold Population

Search sensitivity is quantified via the time-volume product $(VT)$, estimated through injection campaigns across the mass parameter space. The catalog's sensitivity is highest for $100\,M_\odot + 100\,M_\odot$ binaries, with pipeline-dependent variations. Among 1253 subthreshold candidates ($p_\mathrm{astro} < 0.5$, FAR $< 2\,\mathrm{d}^{-1}$), a conservative lower bound estimates $\sim16$ true astrophysical signals, highlighting the potential for further discoveries in archival data.

Data Quality, Glitch Mitigation, and Validation

Rigorous event validation is performed, including glitch identification and mitigation via BAYESWAVE and frequency band excision. Candidates with evidence of instrumental origin (e.g., GW230630_070659) are retained in the catalog but excluded from source property estimation. Time-frequency spectrograms are provided for selected events to illustrate data quality assessments.

Localization and Multimessenger Prospects

Sky localization is limited by the two-detector network during O4a, resulting in larger uncertainties compared to previous runs with Virgo participation. The best localized event is GW230627_015337 ($110\,\mathrm{deg}^2$ at 90% credible level). No confident multimessenger counterparts were identified for O4a candidates, despite rapid public alerts and follow-up efforts.

Implications and Future Directions

GWTC-4.0 provides an unprecedented dataset for population studies, tests of general relativity, cosmological inference, and searches for exotic phenomena. The catalog's expansion enables refined constraints on the mass and spin distributions of compact objects, the nature of the lower and upper mass gaps, and the formation channels of BBHs and NSBHs. The high-SNR events facilitate precision waveform modeling and strong-field gravity tests.

The ongoing upgrades to the LVK network, including increased detector sensitivity and expanded geographic coverage, will enhance detection rates and localization capabilities. Future observing runs are expected to reveal novel CBC sources (e.g., subsolar-mass binaries), supernovae, cosmic strings, and long-lived signals from rapidly rotating neutron stars. The public release of strain data and analysis products via GWOSC ensures broad accessibility for independent analyses and cross-disciplinary research.

Conclusion

GWTC-4.0 marks a significant advancement in gravitational-wave astronomy, doubling the catalog of CBC events and extending the parameter space of observed sources. The catalog's methodological rigor, comprehensive event validation, and multi-model inference framework establish a robust foundation for future astrophysical and fundamental physics investigations. The continued evolution of the LVK network and data analysis techniques will further elucidate the population and dynamics of compact binaries, with implications for stellar evolution, cosmology, and the nature of gravity.