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

Supermassive black hole binaries are expected to produce a gravitational wave (GW) signal in the nano-Hertz frequency band which may be detected by Pulsar Timing Arrays (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 localization of quasi-circular binaries undergoing adiabatic inspiral. We carry out a systematic investigation of the scaling of the localization accuracy with signal strength and number of pulsars in the PTA. Additionally, we find that source sky localization with the International PTA data release one is vastly superior than what is achieved by its constituent regional PTAs.