Pulsar-timing measurement of the circular polarization of the stochastic gravitational-wave background

TL;DR

This paper proposes a new method to detect and analyze the circular polarization of the stochastic gravitational-wave background using pulsar-timing arrays, potentially revealing the nature of black hole binary populations.

Contribution

It introduces an unbiased real-space estimator for separating intensity and circular polarization in the SGWB, validated through simulations and applicable to future PTA data.

Findings

Circular polarization dipole may be detectable with PTAs.

Estimator effectively separates intensity and polarization contributions.

Detection can inform about the dominance of bright sources in the background.

Abstract

Pulsar-timing arrays (PTAs) are in the near future expected to detect a stochastic gravitational-wave background (SGWB) produced by a population of inspiralling super-massive black hole binaries. In this work, we consider a background that can be anisotropic and circularly polarized. We use the expansion of the intensity and the circular polarization in terms of spherical harmonics and the overlap reduction functions for each term in this expansion. We propose an unbiased real-space estimator that can separate the intensity and circular-polarization contributions of the SGWB to pulsar-timing-residual correlations and then validate the estimator on simulated data. We compute the signal-to-noise ratio of a circular-polarization component that has a dipole pattern under different assumptions about the PTA. We find that a nearly-maximal circular-polarization dipole may be detectable, which can aid in determining whether or not the background is dominated by a handful of bright sources.