Probing anisotropies of gravitational-wave backgrounds with a space-based interferometer. II. Perturbative reconstruction of a low-frequency skymap

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 nonstationarity 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 gravitationalwave 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 for 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.PACS numbers: 04.80.Nn, 95.55.Ym, 95.30.Sf

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