Session B17: What we are Learning from the Population of Detected Binary Black Hole Mergers

The properties of binary black holes observed by LIGO and Virgo

The LIGO–Virgo gravitational-wave detector network has now observed 10 binary black hole coalescences. By comparing the measured signal with model templates we can infer the properties of the source systems. These can be used to constrain the astrophysics of binary black hole formation and evolution. I will review the techniques used to make these inferences, and highlight some of the interesting results obtained from our catalogue of discoveries.   

Rates of Compact Binary Mergers from LIGO-Virgo's First and Second Observing Runs

Estimating the rates of compact binary mergers from gravitational-wave (GW) data produced by the LIGO-Virgo network of ground based detectors is an important science target of the LIGO-Virgo collaboration (LVC), given that such rates could help inform the construction of models of stellar mass binaries. In this talk, we describe the Poisson-statistics-based formalism used to compute these rates. From data acquired during the first and second observing runs (O1 and O2) and analyzed by the GstLAL pipeline, we estimate posterior probability distributions on the astrophysical rates of mergers for three distinct classes of compact binaries, as well as assign, to candidate events, source-specific probabilities of their astrophysical origin. We find the rate of binary black hole mergers and binary neutron star mergers, at 90% confidence, to be 9−100/Gpc^3/yr and 97−3140/Gpc^3/yr, respectively, and a 90% upper limit on the rate of neutron star black hole mergers to be 610/Gpc^3/yr.   

Using Gravitational Waves to Observe the Black Hole Mass Gap due to Pair-Instability Supernovae

It has been suggested that the pair-instability mechanism could prevent the formation of black holes between 50 and 130 solar masses. In this talk, we assess the capability of the Advanced LIGO and Advanced Virgo detectors at design-sensitivity to observe this gap in the black hole mass function. We simulate a population of binary black hole mergers and use Bayesian inference to measure the component masses of the population. The resulting posterior distributions are used to determine if a mass gap can be observed from measurements, and we compare our results to previously observed astrophysical events by ground-based gravitational-wave detectors.   

Characterization of low-significance gravitational-wave compact binary sources

LIGO and Virgo will start their third observing run in early 2019. Given our current rate estimates, we expect the detection of a binary black hole signal every week. The majority of these sources will be far away, and thus have low signal-to-noise ratios (SNRs). In this talk, we show if and to which extent the physical parameters of neutron stars and black holes in binaries can be measured at low SNRs. We find that subthreshold binary neutron stars, with SNRs below 12 (10), yield uncertainties in their sky position larger than 400 (700) deg² (90% credible interval), allowing electromagnetic follow-ups by wide-field/all-sky survey instruments. The luminosity distance, which could be used to measure the Hubble constant with standard sirens, has relative uncertainties larger than 40% for binary neutron stars and neutron star black hole mergers. We will also show that we may be able to recover some information for intrinsic parameters, masses and spins, at SNRs as low as 6, but multimodality is not uncommon and can significantly broaden the posteriors. For spins, in particular, the uncertainty is higher for systems with comparable component masses or lack of spin precession.   

Binary Black Hole gravitational waveform reconstruction with minimal morphological assumptions

The recently published catalog of Gravitational Wave events by the LIGO Virgo Collaboration, GWTC-1, showcases ten Binary Black Hole (BBH) sources whose signal waveforms were recovered using General Relativity (GR) template-based analyses. The catalog also presents signal reconstructions for these events using wavelet-based analyses that make minimal morphological assumptions. These analyses look for short duration, burst-like transients that appear coherently in the data of multiple detectors. Being agnostic to the precise morphology of the waveform, they can in principle offer waveform reconstructions that are robust to modeling errors, offering the potential to verify the presence of higher mode content or deviations from GR, for example. We find the waveforms reconstructed by morphology independent and templated analyses to be consistent within known errors, which lends credence to the inferred parameters and nature of these systems.   

The impact of higher modes for GW170729

GW170729 is a binary black hole merger recently announced by the LIGO Scientific and Virgo Collaborations (LVC). Its large detector-frame mass increases the relative impact of higher harmonics. In the paper announcing the discovery of this signal, the LVC comments on the modest impact of higher modes using analyses available at the time. We present a more thorough reanalysis of this event using the latest models and numerical-relativity waveforms with and without higher modes. We conclude that while the impact of higher modes is small, the inclusion of them shifts the posterior densities for various parameters. We find an increase in support for unequal masses and zero effective spin when higher modes are included for all waveform families.