Session K17: Modeling Populations of Compact Gravitational-Wave Sources

Where are LIGO's Big Black Holes?

In LIGO's O1 and O2, the detectors were sensitive to binary black hole coalescences with component masses up to $100\,M_\odot$, with binaries with primary masses above $40\,M_\odot$ representing $>90\%$ of the total accessible sensitive volume. Nonetheless, of the 5.9 detections reported by LIGO-Virgo, the most massive component black hole was only $\sim36\,M_\odot$. We argue that the absence of detected binary systems with component masses heavier than $\sim40\,M_\odot$ may be preliminary evidence for an upper mass gap, as predicted by pair-instability supernovae. By allowing for the presence of a mass gap, we find weaker constraints on the shape of the underlying mass distribution of binary black holes. We fit a power-law distribution with an upper mass cutoff to real and simulated BBH mass measurements, finding that the first 3.9 BBHs favor shallow power law slopes $\alpha < 3$ and an upper mass cutoff $M_\mathrm{max} \sim 40\,M_\odot$. This inferred distribution is entirely consistent with the two recently reported detections, GW170608 and GW170814. We show that with $\sim10$ additional LIGO-Virgo BBH detections, fitting the BH mass distribution will provide strong evidence for an upper mass gap if one exists.   

Disentangling the potential dark matter origin of LIGO's black holes

The nature of the dark matter remains an unsolved mystery of physics. LIGO's recent detections of gravitational waves from binary black hole coalescences have restored interest in the possibility that dark matter could be comprised solely of primordial black holes. Astrophysical constraints suggest that primordial black holes of mass comparable to those already detected by LIGO (above $\sim10 M_\odot$) cannot account for all of the dark matter. At lower masses, constraints found by microlensing surveys dominate, though these surveys have claimed conflicting results. We consider a primordial black hole mass distribution that accounts for all of the dark matter while remaining consistent with LIGO's observations arising from primordial black hole binaries and with microlensing constraints. We find that this distribution also makes an interesting prediction: that $\sim 1 \%$ of the black holes LIGO observes will have masses less than the mass of our Sun and $\sim 10 \%$ will exist within the mass gap. Advanced LIGO's enhanced sensitivity will allow a $\sim 50 \%$ chance of detecting 100 binary black hole mergers, and a search for black holes below one solar mass-which otherwise have no known astrophysical formation mechanism-could allow LIGO to pin down the nature of dark matter.   

Compact binary merger rates using mass and spin models

LIGO and Virgo have observed a handful of compact binary mergers -- binary black holes and binary neutron stars -- and measured their masses and (vector) spins. From the standpoint of population statistics, the two most interesting measurables are the merger rate and the mass \& spin distribution. Since our detection efficiency varies as a function of mass and spin, the rates inferred from the data are sensitive to the mass \& spin distribution. However, that distribution is also unknown, and needs to be determined from the data. Here we present work which jointly infers the mass \& spin distributions -- restricted to a parameterized family of models -- along with rates, using a fully coherent Bayesian approach.   

Dynamical Formation of Compact Object Binaries in Globular Clusters

We explore the formation and evolution of binary star systems containing black holes, neutron stars, and white dwarfs in globular clusters (GC). We use Northwestern's Cluster Monte Carlo Code, \texttt{CMC}, to build a set of 137 fully-evolved GC models that, overall, effectively match the properties of the observed GCs in the Milky Way. We explore the applications of these systems to both gravitational-wave and X-ray astronomy. We estimate that, in total, the MW GCs contain $\sim40$ sources which will be detectable by the Laser Interferometer Space Antenna (\textit{LISA}). We predict $\sim10$ of these sources will be BH--BH binaries. Furthermore, we show that some of these BH--BH binaries can have signal-to-noise ratios large enough to be detectable even in GCs in the Andromeda galaxy and the Virgo cluster.   

New estimate of coincident rates between gravitational-wave and short gamma-ray burst observations by Advanced LIGO and Virgo

Short Gamma-Ray Burst (sGRB) progenitors have long been thought to be coalescing compact system of two Neutron Stars (NSNS) or a Neutron Star and a Black Hole (NSBH). The recent detection of a gravitational-wave signal in coincidence with electromagnetic observations confirmed this scenario and provided new physical information on the nature of these astronomical events. More coincident events should reveal paramount features of these coalescences. We use sGRB observations by the Swift satellite to estimate the rate of detectable coincident gravitational-wave and electromagnetic observations by the Advanced LIGO and Advanced Virgo detectors in future observing runs. We present a comprehensive discussion of the factors affecting this estimate.   

Pulsar Timing Constraints on the Fermi Massive Black-Hole Binary Blazar Population

Blazars are thought to be active galactic nuclei whose jets are almost aligned with our line-of-sight. Electromagnetic observations of blazars have found quasi-periodic behavior in their light curves on timescales of order years. One interpretation of such behavior is that the quasi-periodicity is due to the presence of a massive black-hole binary. We test the binary hypothesis of the cosmic blazar population as discovered by the Fermi Gamma-Ray Space Telescope with recent pulsar-timing array upper limits on the stochastic nanohertz gravitational-wave background. We find that the binary interpretation is inconsistent with pulsar-timing upper limits; thus, binarity alone cannot fully explain quasi-periodicity in the cosmic blazar population.   

Studying Hierarchical Triple Galactic Binaries in LISA

The Laser Interferometer Space Antenna (LISA) is expected to be sensitive to tens of millions of binaries in our galaxy, most of which will constitute a noise source. It is expected that tens of thousands of these binaries will be resolvable by LISA and a substantial fraction of those will be part of hierarchical triple systems. I will discuss modeling these systems for LISA, and determine how well we can estimate the parameters of the outer orbit. Furthermore, I will discuss the potential existence and properties of a ``confusion'' regime of parameters space where the triple system could be mis-identified as an isolated binary leading to biased parameter estimation.   

Precision Standard Siren Cosmology

We discuss the constraints on the Hubble constant to be expected from standard siren sources in ground-based gravitational wave detectors. We consider binary neutron star and binary black hole sources, and focus on the role of golden sirens (the loudest and best constrained sources) to constrain cosmological parameters. We consider two approaches: the counterpart case, where electromagnetic observations provide an independent measurement of the redshift to the sources, and the statistical case, incorporating an analysis over all potential host galaxies within the localization volumes. Our analysis includes realistic measurement uncertainties and selection biases. With $\sim10/60/200$ binary neutron star standard sirens with electromagnetic counterparts, $H_0$ would be constrained to $4/2/1\%$. Although the rates, and thus precise timetable, remain uncertain, precision standard siren cosmology can be expected in the foreseeable future.   

Centihertz to Kilohertz: Multi-band GW Astronomy

The expectation of further gravitational wave events from current and upcoming observing runs by LIGO will be greatly enhanced by the space mission LISA. Black hole binaries, with masses similar to GW150914, will be detectable by both experiments, thus providing a unique “multi-band” view of such systems, from the decihertz to the centihertz regime. Simultaneous observations from space and the ground will provide accurate sky localization, mass estimates, and coalescence times. Along with these improvements are enhanced tests of general relativity and cosmological measurements. This talk will overview the potential LIGO, LISA, and third generation detectors will have at (i) extending tests of general relativity and (ii) measuring cosmological parameters using multi-wavelength observations.