Abstract
High-precision timing of an array of millisecond pulsars (MSPs) across the sky can permit the detection of low-frequency gravitational waves (GW) from supermassive black-hole binaries (SMBHBs) and potentially exotic sources such as cosmic strings, constrain dark matter and additional planets within the Solar System, and provide insight into the changing behaviour of the interstellar medium (ISM) on multiple timescales. Pulsar Timing Array (PTA) experiments are underway around the world, with Canadian institutions participating in the North American NANOGrav collaboration using the Arecibo, Green Bank, and JVLA telescopes. The CHIME/Pulsar collaboration has recently begun observing all visible NANOGrav MSPs on a daily basis, and will provide high-cadence monitoring of ISM dispersive and scattering changes to improve the precision timing attainable by NANOGrav. A detection of GW is expected within roughly the next decade. This White Paper reviews the science attainable with PTAs, including current NANOGrav constraints on GW strains and the resulting implications for the SMBHB population and typical environments, and highlighting work on ISM variations, MSP astrometry and spins. Observing requirements, including the discovery of new MSPs, and strategy for future GW detection, are discussed in the context of current and planned telescopes and instrumentation, including the Square Kilometre Array. Recommendations: Canada should participate in the construction, operation, and data dissemination of the Square Kilometre Array at a level that allows Canadian scientists to take on leading roles with the pulsar-related Key Science Projects. Canadian expertise is well-established and our scientists should be given access to such leadership opportunities. Funding should be found to allow the CHIME telescope, including its pulsar instrument, to operate beyond the nominal 5-year lifetime provided for by the initial CFI funds. CHIME/Pulsar has potential to be an extremely valuable addition to worldwide PTA efforts. The Canadian community should find ways to support the existing Arecibo and Green Bank Observatories, through purchase of telescope time, through direct funding of on-site personnel, or through in-kind contributions in the form of instrumentation or software. Funding should be sought to enable large-scale searches for new pulsars, especially MSPs, with telescopes such as CHORD and the SKA. This will require instrumentation (for CHORD at least), software and personnel. If the U.S. component of the NANOGrav collaboration directs funding and effort toward the establishment of a large new telescope with dedicated observing time for PTA science, then a mechanism should be sought for Canadian astronomers to participate at a meaningful level. This could take the form of instrumentation, software or personnel contributions.