On Detecting Nearby Nano-Hertz Gravitational Wave Sources via Pulsar Timing Arrays

TL;DR

This paper investigates how the near-field effects of nearby massive binary black holes influence the detection capabilities of pulsar timing arrays, highlighting the importance of accounting for wave front curvature in gravitational wave searches.

Contribution

It introduces a detailed analysis of near-field effects on PTA detection of nearby GW sources, emphasizing the significance for sources within a few tens of Mpc and proposing detection prospects for specific galactic sources.

Findings

Near-field effects are significant for sources within a few tens of Mpc.

Detectability of Galactic Center MBBH by SKA PTA is feasible.

LMC MBBH detection is challenging but possible with multiple pulsars.

Abstract

Massive binary black holes (MBBHs) in nearby galactic centers, if any, may be nano-Hertz gravitational wave (GW) sources for pulsar timing arrays (PTAs) to detect. Normally the objective GWs for PTA experiments are approximated as plane waves because its sources are presumably located faraway. For nearby GW sources, however, this approximation may be inaccurate due to the curved GW wave front and the GW strength changes along the paths of PTA pulsar pulses. In this paper, we analyze the near-field effect in the PTA detection of nearby sources and find it is important if the source distance is less than a few tens Mpc, and ignoring this effect may lead to a significant signal-to-noise underestimation especially when the source distance is comparable to the pulsar distances. As examples, we assume a nano-Hertz MBBH source located at either the Galactic Center (GC) or the Large Magellanic Cloud (LMC) according to the observational constraints/hints on the MBBH parameter space, and estimate its detectability by current/future PTAs. We find that the GC MBBH may be detectable by the Square Kilometer Array (SKA) PTA. It is challenging for detecting the LMC MBBH; however, if a number ($N\gtrsim10$) of stable millisecond pulsars can be found in the LMC center, the MBBH may be detectable via a PTA formed by these pulsars. We further illustrate the near-field effects on the PTA detection of an isotropic GW background contributed mainly by nearby GW sources, and the resulting angular correlation is similar to the Hellings-Downs curve.