# Constraining alternative polarization states of gravitational waves from individual black hole binaries using pulsar timing arrays

## TL;DR

This paper investigates the potential of pulsar timing arrays to detect or constrain alternative gravitational wave polarization states predicted by modified gravity theories, focusing on black hole binary signals and their phase evolution.

## Contribution

It develops a theory-agnostic framework and software extension to identify deviations from General Relativity in pulsar timing data, including multiple polarization modes.

## Key findings

- Pure dipole signals can be partially distinguished from quadrupole signals.
- Full polarization models reduce confusion between different gravitational wave modes.
- Forecasts set upper limits on alternative polarization amplitudes based on current noise levels.

## Abstract

Pulsar timing arrays are sensitive to gravitational wave perturbations produced by individual supermassive black hole binaries during their early inspiral phase. Modified gravity theories allow for the emission of gravitational dipole radiation, which is enhanced relative to the quadrupole contribution for low orbital velocities, making the early inspiral an ideal regime to test for the presence of modified gravity effects. Using a theory-agnostic description of modified gravity theories based on the parametrized post-Einsteinian framework, we explore the possibility of detecting deviations from General Relativity using simulated pulsar timing array data, and provide forecasts for the constraints that can be achieved. We generalize the {\tt enterprise} pulsar timing software to account for possible additional polarization states and modifications to the phase evolution, and study how accurately the parameters of simulated signals can be recovered. We find that while a pure dipole model can partially recover a pure quadrupole signal, there is little possibility for confusion when the full model with all polarization states is used. With no signal present, and using noise levels comparable to those seen in contemporary arrays, we produce forecasts for the upper limits that can be placed on the amplitudes of alternative polarization modes as a function of the sky location of the source.