First-hand information for everyone
Scientists working with the LIGO, Virgo and KAGRA collaborations have released GWTC-5.0, an updated catalog of gravitational-wave candidates from the second part of the fourth observing run (O4b). Gravitational waves are ripples in space and time produced when two very dense objects—usually black holes or neutron stars—orbit each other and merge. The catalog adds many new merger candidates and gives more detailed measurements for a subset of the clearest events.
The team searched data taken between 2024 April 10 and 2025 January 28, plus a four-day engineering run just before O4b. The detectors that contributed were the two LIGO instruments (Hanford and Livingston) and Virgo. KAGRA did not take part in this observing period. The analysts ran several search pipelines, both in near-real time and later with improved data cleaning. They report 1,886 initial triggers with a low false-alarm threshold, and from these identify 161 compact binary coalescence candidates that have at least a 50% probability of being astrophysical (p_astro ≥ 0.5) and that passed validation checks.
For 104 of those candidates the team could measure source properties with higher confidence (false-alarm rate < 1 per year). All of the new candidates are consistent with binary black hole mergers; they did not find new binary neutron star or neutron-star–black-hole signals in O4b. The catalog shows a wide range of black hole masses. Median component masses in the new events run from about 5.14 times the Sun’s mass (GW241109_115924) up to about 70 solar masses (GW241116_151753). Some events are much louder than usual. Five binaries in O4b had a network signal-to-noise ratio (SNR) above 30, and the loudest so far, GW250114_082203, had SNR 76.9. A loud signal like that gives a more precise sky position (for GW250114_082203 the 90% credible area was about 42 square degrees) and tighter measurements of the system’s properties.
These results matter because a larger and clearer catalog lets astronomers study how black holes form and how they spin. For example, several sources show a measurable aligned spin parameter (called chi_eff) that is likely different from zero. The team gives two examples: GW241011_233834 with chi_eff ≈ 0.50 (+0.06/−0.04) and GW241110_124123 with chi_eff ≈ −0.31 (+0.23/−0.18). Better localization of sources has also improved: GW240615_113620 is the best-localized to date, with a 90% credible area of about 6 square degrees. In one case (GW240925_005809) a high-SNR signal helped verify aspects of the detectors’ calibration inside the observatory.
There are important caveats. Different search pipelines and the choice between online (real-time) and offline (reprocessed) analyses can give different estimates of detection significance. The catalog’s p_astro and false-alarm-rate numbers depend on how each pipeline models noise and on assumptions about how common different types of binaries are. Some candidates are marginal and their significance can change after reanalysis. No confirmed electromagnetic counterparts (light signals) were reported for O4b candidates. Detector uptime also limits what was seen: the three detectors together were taking science-quality data about 30% of the time and two-detector combinations about 70% of the time. Combining these results with earlier updates brings the cumulative catalog to 390 candidates with p_astro ≥ 0.5, growing the sample for future studies and tests of gravity.