Gravitational wave astronomy of single sources with a pulsar timing array

TL;DR

This paper explores the potential of pulsar timing arrays to detect and characterize individual gravitational wave sources from supermassive black hole binaries, including measurement accuracy and the importance of the pulsar term.

Contribution

It provides a comprehensive analysis of detecting single GW sources with PTAs, incorporating pulsar distance measurement via timing parallax and the significance of the pulsar term.

Findings

PTAs can detect GWs from supermassive black hole binaries at low redshift.

Detection is possible up to less than 10^5 years before merger.

Accurate source localization requires considering the pulsar term.

Abstract

Abbreviated: We investigate the potential of detecting the gravitational wave from individual binary black hole systems using pulsar timing arrays (PTAs) and calculate the accuracy for determining the GW properties. This is done in a consistent analysis, which at the same time accounts for the measurement of the pulsar distances via the timing parallax. We find that, at low redshift, a PTA is able to detect the nano-Hertz GW from super massive black hole binary systems with masses of $\sim10^8 - 10^{10}\,M_{\sun}$ less than $\sim10^5$\,years before the final merger, and those with less than $\sim10^3 - 10^4$ years before merger may allow us to detect the evolution of binaries. We derive an analytical expression to describe the accuracy of a pulsar distance measurement via timing parallax. We consider five years of bi-weekly observations at a precision of 15\,ns for close-by ($\sim 0.5 - 1$\,kpc) pulsars. Timing twenty pulsars would allow us to detect a GW source with an amplitude larger than $5\times 10^{-17}$. We calculate the corresponding GW and binary orbital parameters and their measurement precision. The accuracy of measuring the binary orbital inclination angle, the sky position, and the GW frequency are calculated as functions of the GW amplitude. We note that the "pulsar term", which is commonly regarded as noise, is essential for obtaining an accurate measurement for the GW source location. We also show that utilizing the information encoded in the GW signal passing the Earth also increases the accuracy of pulsar distance measurements. If the gravitational wave is strong enough, one can achieve sub-parsec distance measurements for nearby pulsars with distance less than $\sim 0.5 - 1$\,kpc.