Optimal Strategies for Characterizing the Gravitational Wave Background with Pulsar Timing Arrays

Gravitational-waves are ripples in the fabric of space-time, generated by the acceleration of mass. It is theorized that binary supermassive black holes (SMBH) at the centers of merging galaxies should generate gravitational-waves at nanohertz frequencies. The aggregate gravitational radiation from all of these binaries across the observable universe should form a stochastic gravitational-wave background (GWB). Such a background is detectable via high-precision timing observations of a set of “recycled” millisecond pulsars (MSPs), an approach known as a pulsar timing array (PTA).

MSPs act as stable, naturally occurring clocks; this property allows them to be used as gravitational-wave detectors. Gravitational-waves from SMBH passing through our galaxy will cause the pulsar-Earth light travel time to oscillate on periods of years. PTAs search for gravitational-waves by looking for correlations in the pulse times of arrival (TOAs) of many pulsars. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is expected to see increasing evidence for a GWB over the next few years. Continued observations after this point will result in a detailed characterization of the GWB spectrum, allowing us to distinguish between a SMBH signal and other proposed sources such as cosmic strings. Eventually we expect to be able to resolve signals from individual SMBH binaries, providing additional astrophysical insight into these systems.

PTA sensitivity to a GWB is a function of telescope capabilities, and the properties and number of MSPs observed. Therefore, the time it takes to achieve PTA science results will be determined, in part, by how consistently new MSPs can be added to the PTA and how efficiently current and future telescope resources are allocated to timing these MSPs. I discuss some ongoing efforts to optimize PTA sensitivity through modeling the MSP population and analysis of the contribution of various observing parameters to the pulsar timing noise budget. I also project future telescopes’ contributions to PTA science and suggest some strategies for making the most of these resources.