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/KAGRA 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).

(Source code)

Timeline for sending gravitational-wave alerts

Beginning from 1 minute before the GW merger time, an early warning search may find a pre-merger candidate. If it does, then an Early Warning alert may be sent before the GW merger time.

Within 1–10 minutes after GW trigger time, the first and second preliminary alerts 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 trigger time of any significant gravitational-wave alerts (possibly within 4 hours for BNS or NSBH sources), the Initial or Retraction alert 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 data analysts. The candidate will either be confirmed by an Initial alert and circular or withdrawn by a Retraction alert and circular if the data quality is unsuitable.

Update alerts 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 may promote a candidate to be a significant gravitational-wave alert if it is compellingly associated with a multimessenger signal (e.g. GRB, core-collapse SN). In this case, Initial alert and circulars will be distributed.

Alert Threshold

Automated preliminary alerts are sent out with an expected false alarm rate (FAR) of \(1.62 \times 10^{-4}\) Hz (fourteen per day) before considering trials factor. We will divide gravitational-wave alerts into two categories: (1) low-significance gravitational-wave alerts and (2) significant gravitational-wave alerts. Significant gravitational-wave alerts will be sent out with an expected false alarm rate of \(3.9 \times 10^{-7}\) Hz (one per month) for CBC sources, and of \(3.2 \times 10^{-8}\) Hz (one per year) for unmodeled burst signals that do not have a CBC signature. This alert threshold aims to reach 90% purity for significant gravitational-wave alerts.

We will not perform any further analysis following low-significance gravitational-wave preliminary alerts or early warning alerts that are not followed by a significant preliminary alert. That means that the initial, retraction, and update alerts will be sent out only for those alerts that are associated with significant preliminary alerts.

Alert Threshold Trials Factor

We run both CBC-focused and burst-focused searches. For CBC, there are four independent searches; for burst, there are three independent searches. There is also an external coincidence search RAVEN that looks at results from both the aforementioned CBC and burst searches. To account for the trials factor from the different searches with statistically independent false alarms, an alert will be marked as significant for events that generate a preliminary alert with a FAR threshold of \(7.7 \times 10^{-8}\) Hz (one per 5 months) for CBC target searches and of \(7.9 \times 10^{-9}\) (one per four years) for unmodeled burst searches. That corresponds to the expected effective rate of significant false alarms of \(3.9 \times 10^{-7}\) Hz (one per month) for CBC sources, and \(3.2 \times 10^{-8}\) Hz (one per year) for unmodeled burst sources.

The presence of a trial factor (eight searches) implies that public alerts will be sent for all triggers that pass a false alarm rate (FAR) threshold of \(2.3 \times 10^{-5}\) Hz (two per day).