From Bright Binaries To Bumpy Backgrounds: Mapping Realistic Gravitational Wave Skies With Pulsar-Timing Arrays

TL;DR

This paper develops fast sky-mapping methods to analyze anisotropic gravitational-wave backgrounds from supermassive binary black holes using pulsar-timing arrays, highlighting challenges in distinguishing anisotropy.

Contribution

It introduces novel, modular sky-mapping strategies for localizing anisotropic gravitational-wave signals and modeling the background, improving analysis of pulsar-timing array data.

Findings

Power anisotropy detection requires SNR > 10.

Modeling with multiple point sources is most interpretable.

Spherical-harmonic modeling best discriminates isotropy.

Abstract

Within the next several years, pulsar-timing array programs will likely usher in the next era of gravitational-wave astronomy through the detection of a stochastic background of nanohertz-frequency gravitational waves, originating from a cosmological population of inspiraling supermassive binary black holes. While the source positions will likely be isotropic to a good approximation, the gravitational-wave angular power distribution will be anisotropic, with the most massive and/or nearby binaries producing signals that may resound above the background. We study such a realistic angular power distribution, developing fast and accurate sky-mapping strategies to localize pixels and extended regions of excess power while simultaneously modeling the background signal from the less massive and more distant ensemble. We find that power anisotropy will be challenging to discriminate from isotropy for realistic gravitational-wave skies, requiring SNR $>10$ in order to favor anisotropy with $10:1$ posterior odds in our case study. Amongst our techniques, modeling the population signal with multiple point sources in addition to an isotropic background provides the most physically-motivated and easily interpreted maps, while spherical-harmonic modeling of the square-root power distribution, $P(\hat\Omega)^{1/2}$, performs best in discriminating from overall isotropy. Our techniques are modular and easily incorporated into existing pulsar-timing array analysis pipelines.