List of gravitational wave observations

This page contains a list of observed/candidate gravitational wave events.

Origin and nomenclature
Direct observation of gravitational waves, which commenced with the detection of an event by LIGO in 2015, plays a key role in gravitational wave astronomy. LIGO has been involved in all subsequent detections to date, with Virgo joining in August 2017.

Joint observation runs of LIGO and VIRGO, designated "O1, O2, etc." span many months, with months of maintenance and upgrades in-between designed to increase the instruments sensitivity and range. Within these run periods, the instruments are capable of detecting gravitational waves.

The first run, O1, ran from September 12, 2015, to January 19, 2016, and succeeded in its first gravitational wave detection. O2 ran for a greater duration, from November 30, 2016, to August 25, 2017. O3 began on April 1, 2019, which was briefly suspended on September 30, 2019, for maintenance and upgrades, thus O3a. O3b marks resuming of the run and began on November 1, 2019. Due to the COVID-19 pandemic O3 was forced to end prematurely. O4 began on May 24, 2023; initially planned for March, the project needed more time to stabilize the instruments.

The O4 observing run has been extended from one year to 18 months, following plans to make further upgrades for the O5 run. Updated observing plans are published on the official website, containing the latest information on these runs. There is a two month commissioning break planned from January to March 2024, after which observations will resume for the remainder of O4.

Gravitational wave events are named starting with the prefix GW, while observations that trigger an event alert but have not (yet) been confirmed are named starting with the prefix S. Six digits then indicate the date of the event, with the two first digits representing the year, the two middle digits the month and two final digits the day of observation. This is similar to the systematic naming for other kinds of astronomical event observations, such as those of gamma-ray bursts.

Probable detections that are not confidently identified as gravitational wave events are designated LVT ("LIGO-Virgo trigger"). Known gravitational wave events come from the merger of two black holes (BH), two neutron stars (NS), or a black hole and a neutron star (BHNS). Some objects are in the mass gap between the largest predicted neutron star masses (Tolman–Oppenheimer–Volkoff limit) and the smallest known black holes.

List of gravitational wave events
List of binary merger events



Candidate events and marginal detections
There is possible detection of nanohertz waves by observation of the timing of pulsars, but they have not been confirmed at the 5 sigma level of confidence,.

Marginal detections from O1 and O2
In addition to well-constrained detections listed above, a number of low-significance detections of possible signals were made by LIGO and Virgo. Their characteristics are listed below, only including detections with a <50% chance of being noise:

Observation candidates from O3/2019
From observation run O3/2019 on, observations are published as Open Public Alerts to facilitate multi-messenger observations of events. Candidate event records can be directly accessed at the Gravitational-Wave Candidate Event Database (GraceDB). On 1 April 2019, the start of the third observation run was announced with a circular published in the public alerts tracker. The first O3/2019 binary black hole detection alert was broadcast on 8 April 2019. A significant percentage of O3 candidate events detected by LIGO are accompanied by corresponding triggers at Virgo.

False alarm rates are mixed, with more than half of events assigned false alarm rates greater than 1 per 20 years, contingent on presence of glitches around signal, foreground electromagnetic instability, seismic activity, and operational status of any one of the three LIGO-Virgo instruments. For instance, events S190421ar and S190425z weren't detected by Virgo and LIGO's Hanford site, respectively.

The LIGO/Virgo collaboration took a short break from observing during the month of October 2019 to improve performance and prepare for future plans, with no signals detected in that month as a result.

The Kamioka Gravitational Wave Detector (KAGRA) in Japan became operational on 25 February 2020, likely improving the detection and localization of future gravitational wave signals. However, KAGRA does not report their signals in real-time on GraceDB as LIGO and Virgo do, so the results of their observation run will likely not be published until the end of O3.

The LIGO-Virgo collaboration ended the O3 run early on March 27, 2020, due to health concerns from the COVID-19 pandemic.

List of unconfirmed O3 event alerts

Observation candidates from O4/2023
On 15 June 2022, LIGO announced to start the O4 observing run in March 2023. As the date got closer, engineering challenges delayed the observing run to May 2023. An engineering run to assess the sensitivity of LIGO, Virgo, and KAGRA began in April, with the Hanford detector's first operations beginning on April 29, and the Livingston and Virgo detectors' first operations beginning on May 5.

On March 7, 2023, a gamma-ray burst compatible with a neutron star merger was detected by the Fermi telescope and named GRB 230307A. The burst, identified as being from a host galaxy approximately 296 Mpc away, would likely have only been marginally detected at best by LIGO if it had been operating at the time, as the detectors would only later achieve a sensitivity of 160 Mpc for neutron star mergers by O4's beginning, 3 months later.

Near the end of the engineering run on 15 May 2023, LIGO announced that O4 would be beginning on 24 May 2023, running for 20 months with up to 2 months of maintenance. The LIGO detectors initially failed to achieve the hoped for 160-190 Mpc sensitivity for neutron star mergers, but did achieve an improved 130-150 Mpc sensitivity over O3's 100-140 Mpc, later improving to nearly 160 Mpc for both detectors by late 2023. Virgo was found to have both a damaged mirror and other new, unknown noise sources, limiting its sensitivity to just 31-35 Mpc (similar to its performance during O2 in 2017, and lower than O3's 40-50 Mpc.) As a result, Virgo spent most of 2023 in commissioning, with a deadline of March 2024 to improve its sensitivity before joining O4. KAGRA achieved its planned 1 Mpc sensitivity before returning to commissioning in July, with plans to rejoin at an improved 10 Mpc sensitivity by early 2024. However, the 7.5 2024 Noto earthquake occurred on 1 January 2024 only 103 km from KAGRA, damaging the detector's sensitive instruments and delaying its development by at least several months.

On 18 May 2023, near the end of the engineering run and shortly before O4 proper, the first candidate gravitational wave event was detected. Four more were detected before the official beginning of the run. In October, LIGO announced a planned pause between January and March 2024, for a mid-run commissioning break intended to reduce noise and improve the uptime of the detectors.

The O4b run began in April 2024 with the addition of the Virgo detector at a sensitivity of 55 Mpc. The Livingston detector achieved an increased sensitivity of 170-175 Mpc, while the Hanford detector maintained its pre-break sensitivity of 155-160 Mpc. Due to a variety of factors including delays in technologies required for O5, the decision was made in June 2024 to extend O4 by several months to June 2025, with O5 expected to begin in late 2027 or early 2028.

List of O4 event alerts