Superevents are an abstraction to unify gravitational-wave candidates from multiple search pipelines. Each superevent is intended to represent a single astrophysical source.
A superevent consists of one or more event candidates, possibly from different pipelines, that are clustered based on coalescence time for modeled searches, and trigger time for unmodeled searches.
While the initial clustering window for superevents is 1 second, this window can expand as event candidates are added. Concretely, if the time of the first event is \(t\), the window for the superevent is \((t - 1, t + 1)\). If the next event candidate arrives at \(t + 0.1\), the window becomes \((t - 1, t + 1.1)\). In this way, the superevent windows can become larger than 1 second.
At any given time, one event belonging to the superevent is identified as the preferred event. The superevent inherits properties from the preferred event such as time, significance, sky localization, and classification.
The superevent accumulates event candidates from the search pipelines and updates its preferred event as more significant event candidates are reported (see Selection of the Preferred Event). The name of the superevent does not change. The naming scheme is described in the alert contents section. Once a preferred event candidate passes the public alert threshold (see Alert Threshold), it is frozen and a preliminary alert is queued using the data products of this preferred event. New event candidates are still allowed to be added to the superevent as the necessary annotations are completed. Once the preliminary alert is received by the GCN broker, the preferred event is revised after a timeout and a second preliminary notice is issued. Note that the latter is issued even if the preferred event candidate remains unchanged.
Selection of the Preferred Event¶
When multiple online searches report events at the same time, the preferred event is decided by applying the following rules, in order:
A publishable event, meeting the public alert threshold, is given preference over one that does not meet the threshold.
In the case of multiple CBC events, three-interferometer events are preferred over two-interferometer events, and two-interferometer events are preferred over single-interferometer events.
In the case of multiple CBC events with the same number of participating interferometers, the event with the highest SNR is preferred. The SNR is used to select the preferred event among CBC candidates because higher SNR implies better sky location and parameter estimates from low-latency searches. In the case of multiple Burst events, the event with the lowest FAR is preferred.
See also the preferred event selection flow chart in our software documentation.
Per Pipeline Event Information¶
For many GW candidates, multiple pipelines will produce at least one event. The Per Pipeline Event Information table on GraceDB for superevents associated with a significant alert displays properties of the highest SNR event from each search pipeline. Specifically, we provide the estimated GW arrival time and significance (FAR). In general, we expect significant CBC candidates to be identified in more than one pipeline, though there may be exceptions for events seen in only a single detector.