New binary black hole mergers in the LIGO--Virgo O3a data (2201.02252v3)

Abstract: We report the detection of ten new binary black hole (BBH) mergers in the publicly released data from the the first half of the third observing run (O3a) of advanced LIGO and advanced Virgo. We identify candidates using an updated version of the IAS search pipeline and compile a catalog of signals that pass a significance threshold of astrophysical probability greater than 0.5 (following the GWTC-2.1 and 3-OGC catalogs). The updated IAS pipeline is sensitive to a larger region of parameter space, applies a template prior that accounts for different search volume as a function of intrinsic parameters, and uses an improved coherent detection statistic that optimally combines the data from the Hanford and Livingston detectors. Among the ten new events, we observe interesting astrophysical scenarios including sources with confidently large effective spin parameters in both the positive and negative directions, high-mass black holes that are difficult to form in stellar collapse models due to (pulsational) pair instability, and low-mass mergers that bridge the gap between neutron stars and the lightest observed black holes. We infer source parameters in the upper and lower black hole mass gaps with both extreme and near-unity mass ratios, and one of the possible neutron star--black hole mergers is well localized for electromagnetic counterpart searches. We detect all of the GWTC-2.1 BBH mergers with coincident data in Hanford and Livingston except for three loud events that get vetoed, which is compatible with the false-positive rate of our veto procedure, and three that fall below the detection threshold. We also return to significance the event GW190909_114149, which was reduced to a sub-threshold trigger after its initial appearance in GWTC-2. This amounts to a total of 42 BBH mergers detected by our pipeline's search of the coincident Hanford--Livingston O3a data.

Overview of "New Binary Black Hole Mergers in the LIGO--Virgo O3a Data"

The paper "New Binary Black Hole Mergers in the LIGO--Virgo O3a Data" reports the detection of ten new binary black hole (BBH) mergers using data from the first half of the third observing run (O3a) of advanced LIGO and Virgo. The authors employed an updated version of the IAS pipeline, which is sensitive to a broader parameter space and includes improvements in coherent detection statistics, allowing for optimal combination of data from the Hanford and Livingston detectors. This updated methodology enhances the capability to detect BBH mergers across a wider range of intrinsic parameters.

Key Experimental and Analysis Methodologies

1. Template Bank Construction: The updated pipeline expands the frequency range to 24--600 Hz to capitalize on improved high-frequency sensitivity compared to previous observing runs. A new bank has been created for high-mass mergers, covering mass ranges that may include intermediate mass black holes (IMBHs).
2. Signal Processing Enhancements: Improvements in line-flagging procedures have been implemented, reducing spectral leakage effects by iterating through regional flagging based on amplitude spectral density.
3. Coherent Detection Score: The derivation of a new multi-detector statistic maximizes signal hypothesis information, coherently integrating matched-filter timeseries from each detector to approximate Bayesian evidence.
4. Template Prior: Transitioning from a uniform geometric template prior to one that is uniform in constituent masses and effective spins enhances sensitivity in parameter regions less populated by templates, thus aligning more closely with the astrophysical parameter space distribution.
5. Astrophysical Probability ($p_{\rm astro}$) Calculation: The probability of astrophysical origin is computed using updated methods that estimate density distributions of triggers, allowing integration over signal and noise hypotheses.

Results and Astrophysical Implications

The detection of ten new BBH mergers in the O3a data highlights several intriguing astrophysical scenarios:

• High-Mass Black Holes: Several events show constituent black hole masses in ranges challenging to form under standard stellar collapse models due to pulsational pair instability. This might suggest hierarchical formations or alternative exotic processes.
• Effective Spin Observations: Some mergers display large effective spins, potentially indicating dynamical formation channels where spin orientations are randomized due to interactions in dense stellar environments.
• Mass Gap Bridging: Discoveries include low-mass mergers, edging into observed gaps between neutron stars and lightest black holes, providing empirical insights into the continuous nature of mass distributions potentially supported by formation channel dynamics.

Significance and Future Directions

The implications of these findings contribute to ongoing investigations into BBH formation channels, mass distribution features such as the upper and lower mass gaps, and effective spin impacts within the BBH population. Predictive modeling of formation channels now requires considering possible hierarchical merger scenarios and rapid accretion processes.

Additionally, the paper suggests that the upcoming analyses of O3b data should incorporate Virgo data in coincident searches, explore precessing and higher-harmonic waveform models, and refine methods for detecting high-mass mergers given limitations in frequency bandwidth sensitivity.

In summary, the paper marks a progression in gravitational wave detection capabilities and methodologies, proposing practical steps toward refining theoretical models of binary black hole formation and dynamics in the universe. As gravitational wave astrophysics evolves, these detections symbolize foundational steps toward resolving outstanding questions regarding stellar evolution and compact object interactions at extreme scales.