Pulsar timing arrays (PTAs) are kiloparsec-scale gravitational wave detectors composed of many millisecond pulsars (MSPs) timed over many years. Correlations in the variations in pulse arrival times that are found to be consistent with a quadrupolar signature would indicate the presence of a gravitational wave (GW) signal in the timing residual data. For example, MSP pairs’ timing residuals would follow the Hellings-Downs correlation in the presence of an isotropic stochastic GW background (GWB) produced by a population of coalescing supermassive black hole binaries (SMBHBs), the GW signal likely to be detected first by PTAs.
GW detection by PTAs requires timing many tens of MSPs with ~10-100 ns timing precision. Adequate sky coverage is also very important for both GWB detection, such that the Hellings-Downs curve is sufficiently populated with MSP pairs, and to increase the likelihood of detecting continuous waves (CWs) from individual SMBHBs. Finally, the cadence with which we observe MSPs is important, particularly for CWs: while roughly monthly observations of many MSPs are sufficient for GWB detection, sensitivity to CWs is increased with higher-cadence observations, particularly of MSPs with the highest timing precision.
In this talk, I will focus primarily on the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA and the International Pulsar Timing Array (IPTA). I will describe the facilities and observational strategies employed by these PTAs; the timing analysis methods used especially by NANOGrav to achieve the necessary timing precision, with emphasis on developments in the 12.5- and 15-year data sets that have improved the data quality and increasingly automated the timing procedure; and the process by which data from individual PTAs are combined to form a single IPTA dataset for maximal GW sensitivity. I will also discuss ongoing and thus-far successful efforts to continually expand the number of MSPs timed by these PTAs, including large sky surveys, targeted searches of MSP-like gamma-ray sources, and searches for MSPs in sky regions that would optimally improve PTAs’ sensitivity to both a GWB and individual CW sources.