Gravitational wave events are named starting with the prefix GW, while observations that trigger a event alert but have not been confirmed are named starting with the prefix S. The next two numbers indicate the year the event was observed, the middle two numbers are the month of observation and the final two numbers are the day of the month on which the event was observed. 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. Known gravitational wave events come from the merger of two black holes, two neutron stars, or a black hole and a neutron star. 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.
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. 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 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.
