Gravitational Wave Physics and Astronomy

With the detection of gravitational waves we are beginning to write a new chapter in the annals of astronomy and astrophysics, our experimental understanding of gravity, and our knowledge of the cosmos. This award funds the development of tools and techniques whose use can improve the efficiency and science-return of an important class of gravitational wave detectors, referred to as pulsar timing arrays. Gravitational wave detection via a pulsar timing array involves the accurate and precise monitoring of the radio signals from many different examples of a particular type of astronomical object - pulsars - over periods of many years. Gravitational wave detection via pulsar timing is opportunistic: we have available to us only those pulsars that Nature has provided. The sensitivity of a pulsar timing array to gravitational waves depends on the number of pulsars that can be monitored with sufficient accuracy and precision, and the relative direction of the different pulsars on the sky. The resources available to monitor the pulsars are limited; additionally, the available resources devoted to the timing of each pulsar can influence significantly the accuracy and precision with which it can be monitored. By appropriately choosing the pulsars that are monitored and the time spent monitoring each the sensitivity of the pulsar timing array can be maximized, leading to more rapid first detection of gravitational waves and a more informative scientific product from the effort. Pulsar timing arrays (PTAs) are the only means by which the gravitational waves associated with truly supermassive (on order of a billion or more solar masses) black hole binaries, which are associated with the merger of many galaxies, can be detected. Observation of these binaries will provide important clues to the formation of structure in the early universe and the evolution of the galaxies that are observed today. The sensitivity of a PTA depends on the timing noise associated with each pulsar, the distribution of pulsar-Earth line-of-sight directions in the array, the cadence with which pulse arrival times are determined (i.e., is an array pulsar timed once every six weeks? once a month? once a week?), and the observing time spent on the pulsar at each observation (e.g., 30 min? an hour? several hours?). This project develops a set of quantitative metrics that can be used to "tune" a pulsar timing array to maximize its science return as characterized by three different measures - quickest detection, best multi-messenger astronomy prospects, and best characterization of gravitational wave source - and applies these metrics to identify what benefits can come from a change in the present observing time allocations associated with pulsar timing for gravitational wave detection.