Optimising gravitational-wave sky maps for pulsar timing arrays

TL;DR

This paper enhances gravitational-wave sky mapping for pulsar timing arrays by deriving a point-spread function, analyzing resolution limits, and improving detection significance with a new scheme tested on realistic simulations.

Contribution

It introduces a refined mapping method that accounts for variable local resolution, improving angular resolution and detection significance in PTA gravitational-wave analysis.

Findings

Derived a point-spread function for PTA angular resolution

Identified limitations of previous resolution estimation methods

Achieved approximately double the detection significance in simulations

Abstract

Pulsar timing arrays (PTAs) have recently reported compelling evidence for the presence of a gravitational-wave background signal. Mapping the gravitational-wave background is key to understanding how it is formed, since anisotropy is a tracer for, for example, a supermassive black hole binary origin. In this work we refine the frequentist regularised gravitational-wave mapping analysis developed in our previous work (as part of the MeerKAT PTA 4.5-year data release). We derive a point-spread function describing the angular resolution of a PTA. We investigate how the point spread function changes for different PTA constellations and determine the best possible angular resolution achievable within our framework. Using simulated data, we demonstrate that previous methods do not capture the actual resolution - especially in regions of the sky with a high density of pulsars. We propose an improved scheme that accounts for a variable local resolution and test it using realistic simulations of the latest MeerKAT dataset. We demonstrate that we are able to identify a continuous gravitational wave signal in a region with good pulsar sky coverage with approximately a factor of two increase in significance compared to our previous method.