List of gravitational wave observations
This is a list of observed/candidate gravitational wave events. Direct observation of gravitational waves, which commenced with the detection of an event by LIGO in 2015, constitutes part of gravitational wave astronomy. LIGO has played a role in all subsequent detections to date, with Virgo joining in August 2017. Nomenclature and timeline. 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 final two digits of the year the  event was observed, two digits for the month and two digits for 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. Observations are made in "runs", three of them so far, with maintenance and upgrades of the detectors made between runs. The first run, O1, ran from 12 September 2015 to 19 January 2016, with O2 from 30 November 2016 to 25 August 2017. O3 began on 1 April 2019; it is divided into O3a, from 1 April to 30 September 2019, and O3b, from 1 November 2019 to 27 March 2020. Suspension of observation during October 2019 was for instrument upgrades and fixes, and cessation in March 2020 was due to the COVID-19 pandemic. The fourth  run, O4, is planned (as of March 2022) to begin in December 2022. List of gravitational wave events. List of binary merger events Candidate events and marginal detections. 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: 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. On 15 November 2021, LIGO announced that "we plan to start the O4 observing run in mid-December 2022". List of O3 event alerts