Detecting Parity-Violating Gravitational Wave Backgrounds with Pulsar Polarization Arrays

TL;DR

This paper addresses detecting parity-violating components of an isotropic stochastic gravitational wave background by combining pulsar timing with pulsar polarimetry. Using geometric optics in a plane-wave GW background, it derives the GW-induced redshift $z$ and polarization rotation angle $\chi$ for EM waves, and models the SGWB with Stokes parameters $I(f)$ and $V(f)$. The authors show that cross-correlations between timing and polarimetry observables yield a parity-odd signal that isolates the circular polarization $V(f)$ while sharing the Hellings-Downs angular pattern with the standard intensity signal, enabling a clean separation of parity-violating information. They provide a sensitivity forecast under optimistic SKA-era parameters, suggesting a projected bound $\Omega^V_{\rm GW} \lesssim 0.012$, offering a valuable parity-violation probe and a null test complementary to existing astrometric and timing analyses.

Abstract

Pulsar timing arrays probe isotropic stochastic gravitational wave (GW) backgrounds in the nanohertz band but are insensitive to its parity-violating component. Motivated by recent progress in pulsar polarization arrays, we study the response of pulsar polarimetry to GWs and evaluate its potential to detect circular polarization in isotropic stochastic GW backgrounds, which characterizes parity violation. Based on geometric optics, we derive the rotation of the polarization of electromagnetic waves induced by propagation through a GW background. We show that the cross-correlation between pulsar timing and polarimetry signals isolates the circular polarization component from the GW intensity, sharing the same Hellings-Downs angular pattern. With future facilities such as the SKA, timing-polarimetry correlations could reach sensitivities to the circular polarization of GWs comparable to those of the current astrometric methods.