Measuring Deterministic and Stochastic Gravitational Waves with Pulsar Timing Array Experiments

Abstract

Pulsar timing arrays (PTAs) are uniquely poised to detect the nanohertz-frequency gravitational waves from supermassive black hole binaries (SMBHBs) formed during galaxy merger. Efforts are underway to observe three species of gravitational signal from these systems: the stochastic ensemble, individual, adiabatic binary inspirals, and bursts with memory. This dissertation discusses all three. A typical Bayesian search for evidence of a stochastic gravitational wave background from the superposition of many unresolvable SMBHB inspirals requires weeks to months to deliver results. This is due in part to the inclusion of inter-pulsar spatial and temporal correlations induced in PTA data by such a signal. By integrating a simplified Bayesian search into an existing frequentist statistic, we are able to create a robust background amplitude estimator that requires minimal CPU time and does not compromise the key information gleaned from a full Bayesian analysis. As PTA sensitivity increases, individual binary inspirals will rise above the stochastic background, promising information about local SMBHBs. PTAs, like the North American Nanohertz Observatory for Gravitational Waves, regularly conduct searches for these single-frequency sources, with the latest results coming from the 9- and 11-year data sets. Although detection is still in the future, results are already informing binary candidate properties. Finally, an SMBHB coalescence is theoretically accessible to PTAs through bursts with memory, a purely General Relativistic phenomenon which imparts a permanent spacetime deformation and affects the coalescence signal amplitude at leading quadrupole order. Simulations parameterized by astrophysical observables from galaxy mergers out to z=3 predict the rates and signal-to-noise ratios for bursts occurring in the PTA-band. Extending the synthesized population to include less massive SMBHBs shows space-based interferometers may also observe this atypical signature.