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).
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 The candidate will either be confirmed by an Initial alert and circular or withdrawn by a Retraction alert and circular if the data quality is unsuitable.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 correspond to superevent with a candidate trigger (from any pipeline) with a false alarm (FAR) of \(2.3 \times 10^{-5}\) Hz (two per day) before considering the trials factor. We divide gravitational-wave alerts into two categories: (1) low-significance gravitational-wave alerts and (2) significant gravitational-wave alerts. The LIGO/Virgo/KAGRA collaboration performs human vetting and further analysis on significant gravitational-wave alerts. These significant alerts indicate the presence of a candidate with a false alarm rate (FAR) of \(7.7 \times 10^{-8}\) Hz (one per five months) for searches that target CBC sources and of \(1.6 \times 10^{-8}\) Hz (one per two years) for burst searches that target generic signals.
Early-Warning alerts correspond to EarlyWarning CBC searches with a candidate false alarm (FAR) of \(1.3 \times 10^{-7}\) Hz (one every three months) and are marked as significant gravitational-wave alerts.
We do 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. We will perform human vetting only for significant alerts. Initial, retraction, and update alerts will be sent out only for superevent associated with significant alerts.
False Alarm Rate for Alerts and Trial Factor¶
The reported false alarm rates do not include the trials factor from using multiple search pipelines. The trials factor is applied so alerts are sent out at the expected false alarm rate, accounting for the number of searches online. At the start of the O4b observational period, we have four independent searches that target CBC sources (GstLAL AllSky, MBTA AllSky, PyCBC AllSky, and cWB BBH) and one independent Burst search that targets generic signals (cWB AllSky). There is also an external coincidence search that targets significant candidates from all online pipelines (RAVEN). We thus conservatively estimate a trial factor of 5 for CBC searches and 2 for Burst searches.
The estimated rate of false alarms marked as significant is \(3.9 \times 10^{-7}\) Hz (one per month) for CBC searches and \(3.2 \times 10^{-8}\) (one per year) for Burst searches.
The estimated rate of false alarms marked as low-significance is \(9.3 \times 10^{-5}\) Hz (eight per day) for CBC searches and \(4.6 \times 10^{-5}\) (four per day) for Burst searches.
The number of searches may change during O4b, impacting the trials factor. If this is the case, the threshold for alerts marked as significant will be adjusted such that the estimated rate of false alarms stays one per month. The threshold for alerts marked as low-significance will remain two per day for each search.