Probing anisotropies of gravitational-wave backgroundswith a space-based interferometer II: Perturbative reconstruction of a low-frequency skymap

TL;DR

This paper introduces a perturbative method for reconstructing low-frequency gravitational-wave background skymaps using space-based interferometers, enabling discrimination of galactic and extragalactic sources despite limited angular resolution.

Contribution

It presents two novel perturbative reconstruction techniques for low-frequency GWB skymaps, utilizing harmonic-Fourier and direct time-series approaches, demonstrated with LISA data.

Findings

Reconstruction methods successfully produce GWB skymaps from simulated LISA data.

The techniques can distinguish galactic from extragalactic gravitational-wave backgrounds.

Angular resolution remains limited but useful for source discrimination.

Abstract

We present a perturbative reconstruction method to make a skymap of gravitational-wave backgrounds (GWBs) observed via space-based interferometer. In the presence of anisotropies in GWBs, the cross-correlated signals of observed GWBs are inherently time-dependent due to the non-stationarity of the gravitational-wave detector. Since the cross-correlated signal is obtained through an all-sky integral of primary signals convolving with the antenna pattern function of gravitational-wave detectors, the non-stationarity of cross-correlated signals, together with full knowledge of antenna pattern functions, can be used to reconstruct an intensity map of the GWBs. Here, we give two simple methods to reconstruct a skymap of GWBs based on the perturbative expansion in low-frequency regime. The first one is based on harmonic-Fourier representation of data streams and the second is based on "direct" time-series data. The latter method enables us to create a skymap in a direct manner. The reconstruction technique is demonstrated in the case of the Galactic gravitational wave background observed via planned space interferometer, LISA. Although the angular resolution of low-frequency skymap is rather restricted, the methodology presented here would be helpful in discriminating the GWBs of galactic origins by those of the extragalactic and/or cosmological origins.