Data Analysis

In this section we describe the different online searches looking for GW signals, the selection and vetting of candidates, and parameter estimation analysis.

When multiple candidates from different pipelines are close enough together in time, they will be considered as originating from the same physical event and will be grouped into a single superevent. See the following pages for technical details.

The timeline for distribution of alerts is described below.

Alert Timeline

Here, we describe the sequence of LIGO/Virgo alerts for a single event that will be distributed through the Gamma-ray Coordinates Network (GCN) via notices and circulars (see the Alert Contents and Sample Code sections for details).

Timeline for sending gravitational-wave alerts

Within 1–10 minutes after GW trigger time, the first and second preliminary notices will be sent fully autonomously. The trigger will be immediately and publicly visible in the GraceDB database. Since the procedure is fully automatic, some preliminary alerts may be retracted after human inspection for data quality, instrumental conditions, and pipeline behavior.

Within 24 hours after the GW trigger time (possibly within 4 hours for BNS or NSBH sources), the Initial or Retraction notice and circular will be distributed. It will include an updated sky localization and source classification. At this stage, the event will have been vetted by human instrument scientists and analysts. The candidate will either be confirmed by an Initial notice and circular or withdrawn by a Retraction notice and circular if the data quality is unsuitable.

Within a day, black hole mergers will be fully vetted by experts and retraction or confirmation status will be reported.

Update notice and circulars are sent whenever the sky localization area or significance accuracy improves (e.g. as a result of improved calibration, glitch removal, or computationally deeper parameter estimation). Updates will be sent up until the position is determined more accurately by public announcement of an unambiguous counterpart. At that point, there will be no further sky localization updates until the publication of the event in a peer-reviewed journal.

At any time, we can promote an extraordinary candidate that does not pass our public alert thresholds if it is compellingly associated with a multimessenger signal (e.g. GRB, core-collapse SN). In this case, Initial notices and circulars will be distributed.

Alert Threshold

Automated preliminary alerts are sent for all events that pass a false alarm rate (FAR) threshold. The FAR threshold is \(3.8 \times 10^{-8}\) Hz (one per 10 months) for CBC searches and is \(7.9 \times 10^{-9}\) (one per 4 years) for unmodeled burst searches. Since there are 4 independent CBC searches and 3 independent burst searches, as well as a coincidence search RAVEN that looks at the results from both the aforementioned CBC and burst searches, the effective rate of false alarms for CBC sources is \(1.9 \times 10^{-7}\) Hz (one per 2 months), and for unmodeled bursts is \(3.2 \times 10^{-8}\) Hz (one per year).