Null stream analysis of Pulsar Timing Array data: localisation of resolvable gravitational wave sources

TL;DR

This paper introduces a Bayesian null-stream method for localizing resolvable gravitational wave sources in Pulsar Timing Array data, demonstrating improved sky localization accuracy especially with combined datasets.

Contribution

It develops a novel null-stream analysis technique for PTA data to accurately localize individual gravitational wave sources, enhancing previous methods.

Findings

Null-streams effectively cancel common GW signals for localization.

Localization accuracy improves with more pulsars and stronger signals.

International PTA data yields superior localization compared to regional PTAs.

Abstract

Super-massive black hole binaries are expected to produce a GW signal in the nano-Hertz frequency band which may be detected by PTAs in the coming years. The signal is composed of both stochastic and individually resolvable components. Here we develop a generic Bayesian method for the analysis of resolvable sources based on the construction of `null-streams' which cancel the part of the signal held in common for each pulsar (the Earth-term). For an array of $N$ pulsars there are $N-2$ independent null-streams that cancel the GW signal from a particular sky location. This method is applied to the localisation of quasi-circular binaries undergoing adiabatic inspiral. We carry out a systematic investigation of the scaling of the localisation accuracy with signal strength and number of pulsars in the PTA. Additionally, we find that source sky localisation with the International PTA data release one is vastly superior than what is achieved by its constituent regional PTAs.