Session P02: Gravitational Waves

Pulsar Timing Arrays: Unveiling The Nanohertz-frequency Gravitational-wave Landscape

Pulsar-timing arrays (PTAs) like the North American Nanohertz Observatory for Gravitational waves (NANOGrav) and the International Pulsar Timing Array are poised to chart the new frontier of gravitational wave discovery within the next several years. At these frequencies, expected sources include supermassive black hole binaries, formed when galaxies merge over cosmic time, and emanating gravitational waves in the nanohertz-frequency sensitivity band of PTAs. I will present exciting new results from recent cutting-edge searches, discuss some milestones on the road to the next decade of PTA discovery, and highlight new techniques to enable the future multi-messenger detection of supermassive black-hole binaries.   

Rapid and Flexible GW Background Characterization using Pulsar Timing Array Data

The nanohertz-frequency gravitational wave background (GWB) may have many sources, from supermassive black hole binary inspirals to cosmic strings and cosmological phase transitions. We would like to rigorously characterize the spectral properties of this GWB by applying Bayesian model selection and parameter estimation to Pulsar Timing Array (PTA) data, allowing us to investigate when a PTA could discriminate between these sources, and constrain model parameters. However current techniques are too slow to practically conduct this research. In this talk, I will present a new method which accelerates parameter estimation and model selection of PTA data for GWB spectral characterization, and which can also be used by the broader community to test their theoretical GWB models.   

The tidal precession effect on the gravitational waves from eccentric double white dwarfs

The tidal effect governs the orbital precession of eccentric binaries at small separations. With the upcoming space-based gravitational wave detector (LISA), we can expect to detect the gravitational wave signals from eccentric double white dwarf systems within our galaxy in the near future. While previous studies mainly focus on the precession effect from the equilibrium component of the tide, we study the full tidal contribution including the dynamical component. In this talk, I will discuss both the resonant and off-resonant dynamical tide precession effects. The former occurs when the orbital frequency or its higher harmonics come near the natural frequency of a pulsation mode of the white dwarf, which can cause a strong precession effect distinct from that of the other contributing factors. We show that these resonant regions can cover about 10% of the frequency space for orbits with close separations. On the other hand, the off-resonant effect causes a small extra orbital precession of up to 20% of the overall amount. This effect degenerates with the uncertainties of the eccentricity within lower mass systems, making it unmeasurable in the gravitational wave signal. For higher mass systems, the radiation reaction effect becomes significant enough to constrain the eccentricity, allowing the measurement of the dynamical tide.

    

Forecasting pulsar timing array sensitivity to anisotropy in the stochastic gravitational wave background

Statistical anisotropy in the nanohertz-frequency gravitational-wave background (GWB) is expected to be detected by pulsar timing arrays (PTAs) in the near future. By developing a frequentist statistical framework that intrinsically restricts the GWB power to be positive, we establish scaling relations for multipole-dependent anisotropy decision thresholds that are a function of the noise properties, timing baselines, and cadences of the pulsars in a PTA. We verify that (i) a larger number of pulsars, and (ii) factors that lead to lower uncertainty on the cross-correlation measurements between pulsars, lead to a higher overall GWB signal-to-noise ratio, and lower anisotropy decision thresholds with which to reject the null hypothesis of isotropy. Using conservative simulations of realistic NANOGrav datasets, we predict that an anisotropic GWB with angular power Cl=1>0.3Cl=0 may be sufficient to produce tension with isotropy at the p=3×10−3 (∼3σ) level in near-future NANOGrav data with a 20~yr baseline. We present ready-to-use scaling relationships that can map these thresholds to any number of pulsars, configuration of pulsar noise properties, and sky coverage. We discuss how PTAs can improve the detection prospects for anisotropy, as well as how our methods can be adapted for more versatile searches.   

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 find that in the 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. Moreover, we are better able to distinguish the morphologies at the chosen reference point when the binary is not close to a morphology transition.   

Spin Orientations of Binary Black Holes at their Formation

One of the important parameters that can provide clues about the formation of binary black hole (BBH) mergers detected by ground-based gravitational wave detectors such as LIGO and Virgo are the spin orientations of the black holes. Specifically, the spin tilts, i.e., the angles between each black hole’s spin vector and the binary’s orbital angular momentum vector can change due to precessional effects as the black holes evolve from a large separation to their merger. The tilts one infers by comparing the signal in the detectors’ sensitive band with theoretical waveforms can thus be significantly different from when the binary originally formed. These tilts at the binary’s formation are well approximated in many cases by evolving the BBH backwards in time to a formally infinite separation. Using the tilts at infinity also places all binaries on an equal footing in analyzing their population properties. In this project, we perform parameter estimation for simulated BBH events and investigate the differences between the tilts one infers directly close to merger and those obtained by evolving back to infinite separation. The simulated observations we consider are selected such that their configurations show particularly large differences in their orientations close to merger and at infinity. While these differences may be buried in the statistical noise at current detection sensitivities, we show that in future plus-era (A+ and Virgo+) detectors, they can be easily distinguished in our inferred distributions.