[Paper Review] GWTC-2.1: Deep Extended Catalog of Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run

The LIGO Scientific Collaboration, The Virgo Collaboration+1430 more|arXiv (Cornell University)|Aug 2, 2021

Pulsars and Gravitational Waves ResearchPhysics and Astronomy243 references196 citations

TL;DR

This paper presents an expanded catalog (GWTC-2.1) of gravitational-wave events from compact binary coalescences detected by LIGO and Virgo during the first half of O3, including deeper searches and reanalysis to enrich the event roster.

ABSTRACT

The second Gravitational-Wave Transient Catalog reported on 39 compact binary coalescences observed by the Advanced LIGO and Advanced Virgo detectors between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. We present GWTC-2.1, which reports on a deeper list of candidate events observed over the same period. We analyze the final version of the strain data over this period with improved calibration and better subtraction of excess noise, which has been publicly released. We employ three matched-filter search pipelines for candidate identification, and estimate the astrophysical probability for each candidate event. While GWTC-2 used a false alarm rate threshold of 2 per year, we include in GWTC-2.1, 1201 candidates that pass a false alarm rate threshold of 2 per day. We calculate the source properties of a subset of 44 high-significance candidates that have an astrophysical probability greater than 0.5. Of these candidates, 36 have been reported in GWTC-2. If the 8 additional high-significance candidates presented here are astrophysical, the mass range of events that are unambiguously identified as binary black holes (both objects $\geq 3M_\odot$) is increased compared to GWTC-2, with total masses from $\sim 14 M_\odot$ for GW190924_021846 to $\sim 182 M_\odot$ for GW190426_190642. The primary components of two new candidate events (GW190403_051519 and GW190426_190642) fall in the mass gap predicted by pair instability supernova theory. We also expand the population of binaries with significantly asymmetric mass ratios reported in GWTC-2 by an additional two events (the mass ratio is less than $0.65$ and $0.44$ at $90\%$ probability for GW190403_051519 and GW190917_114630 respectively), and find that 2 of the 8 new events have effective inspiral spins $χ_\mathrm{eff} > 0$ (at $90\%$ credibility), while no binary is consistent with $χ_\mathrm{eff} < 0$ at the same significance.

Motivation & Objective

• Motivation to improve the completeness and depth of the gravitational-wave event catalog from the first half of the third observing run (O3a).
• Provide a deeper, more complete census of compact binary coalescences detected by the LIGO and Virgo network.
• Refine parameter estimation and selection effects to enable robust population and astrophysical inferences.
• Facilitate future multimessenger follow-up and cross-correlation studies with an extended GW sample.

Proposed method

• Apply deep search and reanalysis techniques to include sub-threshold or previously missed events.
• Integrate data from the LIGO and Virgo detectors to construct a comprehensive catalog.
• Perform consistent parameter estimation and event validation across the expanded dataset.
• Characterize selection effects and completeness to support population inferences.
• Provide standardized metadata and provenance to support downstream analyses.

Experimental results

Research questions

• RQ1 How many compact binary coalescence events are present in the first half of the third observing run when using deeper search methods?
• RQ2 What are the updated source-property estimates ( masses, spins, distances ) for the events in the extended catalog?
• RQ3 How does the deeper catalog alter population inferences and event rate estimates for mergers observed by LIGO and Virgo?
• RQ4 What are the selection effects and completeness characteristics of the GWTC-2.1 catalog compared to previous releases?
• RQ5 How can the extended catalog support future multimessenger and follow-up studies?

Key findings

• GWTC-2.1 provides a deeper extended catalog of compact binary coalescences detected in O3a by LIGO and Virgo.
• The paper consolidates refined parameter estimates and a broader set of events than prior catalogs.
• The extended catalog enables more robust population and rate inferences by accounting for deeper searches.
• The work emphasizes the importance of consistent validation and metadata for reproducibility and downstream use.
• The catalog serves as a resource for multimessenger follow-up and cross-messenger studies in gravitational-wave astronomy.