Session T09: Gravitational Wave Inference of Compact Binary Populations

Model-independent inference of the population properties of compact binary coalescences from gravitational wave observations.

The observation of gravitational waves from multiple compact binary coalescences by the LIGO-VIRGO and KAGRA detector networks has enabled us to infer the underlying distribution of compact binaries across a wide range of masses, spins and redshifts. Studying the population of compact binaries through GWTC-3 has facilitated the exploration of binary formation and evolution channels. In light of the new features found in the mass spectrum of binary blackholes and the uncertainty regarding binary formation models, non-parametric population inference has become increasingly popular. In this work, we develop a data-driven clustering framework that can identify features in the component mass distribution of compact binaries simultaneously with those in the redshift evolution of the merger rate from gravitational wave data in the presence of significant measurement uncertainties, while making no assumption on the functional form of these distributions. We test our model on simulated data and demonstrate the accuracy with which it can re-construct the underlying distributions of component masses and redshift. We also re-analyze public LIGO-Virgo-KAGRA data from events in GWTC-3 using our model and compare our results with those from some alternative parametric and non-parametric population inference schemes. Lastly, we explore the potential generalization of our analysis to being able to infer the population distribution of additional parameters like component spins simultaneously with those of the existing parameters. In this talk, I will summarize the methodology of our inference framework, present the results obtained from the afore-mentioned analyses and discuss the astrophysical implications of said results.   

Classifying the generation and formation channels of dynamically-formed gravitational-wave events

Gravitational-wave astrophysics is rapidly growing and quickly shifting its focus from the characterisation of individual events to the description of the distributions of black holes and neutron stars in the Universe. Refined astrophysical models and state-of-the-art data-analysis techniques are currently being developed to establish whether the binaries observed with current interferometers are formed in isolation or through dynamical interactions between compact objects. Focussing on the latter scenario, and relying on fast-to-generate models for dynamical formation scenarios, I lay out a data-driven pipeline to establish from which channels events that are believed to be dynamical in origin come from. I also comment on how the pipeline can be improved with further astrophysical information, and extended to future detectors.   

One Channel to Rule…Some of Them?Constraining the Origins of Binary Black-Hole Mergers with GWTC-3

The third Gravitational-Wave Transient Catalog (GWTC-3) updates the previous catalog (GWTC-2.1) with signals from the second half of the third observing run (O3b), bringing the total number of confident (False Alarm Rate < 1) detections to 76. These signals originate from mergers of compact object binary systems, primarily binary black hole (BBH) systems, whose astrophysical origin remains relatively unconstrained. We investigate the origins of BBH mergers with the analysis of Zevin et. al (2011.10057), who inferred the branching fractions between five BBH formation channels via Bayesian hierarchical analysis of GWTC-2. While the inclusion of data from O3b affects the inferred underlying and detectable branching fractions, the result that BBH mergers originate from multiple formation channels remains robust. Finally, we investigate potential biases by examining the effect of excluding various channels from this analysis on the recovered branching fractions.   

Constraining Concrete Assumptions for Compact Binary Formation through Isolated Binary Evolution

The predicted population of compact binary mergers varies widely with assumptions about their formation and on specific model parameters, necessitating a careful study of these formation parameters. Using the StarTrack binary evolution code, we simulate populations of compact binaries with varied assumptions about supernova recoil kicks, the amount and specific angular momentum of ejected material during mass transfer, and mass loss rates due to stellar wind in hydrogen-dominated stars. The postprocessing of these simulated merger populations allows us to predict a population of gravitational-wave sources which may be observed in a universe matching those model assumptions. We constrain the space of these formation parameters, demonstrating a method of careful interpolation of marginal likelihoods between expensive simulations.   

Things that might go bump in the night: Assessing structure in the binary black hole mass spectrum

Several bumps and dips in the mass spectrum of merging binary black holes have been identified in the LIGO/Virgo data. But are these features signatures of the astrophysical process that produced the observed binary black holes, or are they just due to statistical fluctuations? I will present an analysis addressing this question, and will discuss the implications of these results for the formation of merging black holes, the possible underlying morphology of the binary black hole mass spectrum, and what parts of this spectrum we can expect to resolve with detections in LIGO/Virgo/KAGRA's upcoming fourth observing run.   

Are LIGO-Virgo-KAGRA's black holes made from smaller black holes? Limits from binary black hole effective spin observations

It has been proposed that some black holes in binary black hole systems are born from “hierarchical mergers;" i.e. earlier mergers of smaller black holes. These hierarchical merger products have spin magnitudes χ~0.7, and, if they are dynamically assembled into binary systems, their spin orientations will be sometimes anti-aligned with the binary orbital angular momentum. In fact, ~16% of binary black holes systems that include hierarchical merger products will have an effective inspiral spin parameter, χ_eff < -0.3. Nevertheless, the LIGO-Virgo-KAGRA gravitational-wave detectors have yet to observe a binary black hole system with χ_eff < -0.2. The absence of observed binary black holes with large, misaligned spins automatically limits how many black holes are made from smaller black holes. I will discuss implications for the formation of the most massive black holes.   

Searching for Structure in the Binary Black Hole Spin Distribution

Combining gravitational wave observations to infer the astrophysical distribution of black hole spins allows us to determine the relative contribution from different formation scenarios to the population. Many previous works have modeled spin population distributions using strongly parametric models, making strong assumptions about the shape of the underlying distribution. The results obtained with such models are only valid if the allowed shape of the distribution is well-motivated (i.e. for astrophysical reasons). In this work, we relax these prior assumptions and model the spin distributions using a more data-driven approach, modeling these distributions with flexible cubic spline interpolants in order to allow for capturing structures that the strongly parametric models cannot. We find that adding this flexibility to the model substantially increases the uncertainty in the inferred distributions, but find a general trend for lower support at high spin magnitude and a spin tilt distribution consistent with isotropic orientations. Additionally, we find that artifacts from unconverged Monte Carlo integrals in the likelihood can manifest as spurious peaks and structures in inferred distributions, mandating the use of a sufficient number of samples when using Monte Carlo integration for population inference.   

Distinguishing binary black hole precessional morphologies with gravitational wave observations

The precessional motion of binary black holes can be classified into one of three morphologies, based on the evolution of the angle between the components of the spins in the orbital plane: Circulating, librating around 0, and librating around pi. All binaries are circulating at infinite separation, but they can transition to a librating morphology as they approach merger. These different morphologies close to merger can be related to the binary's formation channel and are imprinted in the binary's gravitational wave signal. We consider the prospects for distinguishing between the different morphologies close to merger using gravitational waves in the Advanced LIGO/Advanced Virgo network with their plus-era sensitivities. Here we consider fiducial high- and low-mass binaries and use Bayesian model selection to determine which morphology is preferred. We have implemented a fast calculation of the morphology, allowing us to restrict to a specific morphology in our stochastic sampling. We find that in the higher SNR cases with well-measured spin angles (most cases with larger spins) we have overwhelming evidence for the true morphology compared to at least one alternative morphology, and even find moderate evidence in many lower-SNR cases. Moreover, we are better able to distinguish the morphologies at the chosen reference point when the binary is not close to a morphology transition. We find that there is no preference for any morphology for the likely precessing binary black hole signal GW200129_065458.   

Growing Pains: Understanding and Handling Systematic Errors in Gravitational-Wave Population Analyses

During the first three observing runs of gravitational-wave detectors, we observed nearly 100 signals from compact binary mergers. During the fourth observing run, which is expected to begin later this year, we can anticipate observing hundreds more such systems. This expanding population has and will continue to enable many new insights ranging from stellar astrophysics to cosmology and strong-field tests of general relativity. Most of these measurements rely on combining information from the entire catalog of mergers to constrain the underlying physics. However, the methods used scale poorly with the increasing size of the observed population. In this talk, I will demonstrate that existing analysis methods scale poorly with the scaling with population size and discuss potential solutions to enable gravitational-wave astronomy at scale.