Model-independent search for anisotropies in stochastic gravitational-wave backgrounds and application to LIGO-Virgo's first three observing Runs

TL;DR

This paper introduces a new, efficient, model-independent method for mapping anisotropies in stochastic gravitational-wave backgrounds using LIGO-Virgo data, improving the analysis of GW sky maps and spectral shapes.

Contribution

The paper develops a novel, model-independent analysis pipeline for anisotropic stochastic GW backgrounds, including full pixel correlation matrix inversion and spectral shape probing.

Findings

Set new upper limits on GW anisotropies and monopole.

Provided constraints on the spectral shape of stochastic backgrounds.

Demonstrated the pipeline's effectiveness with LIGO-Virgo data.

Abstract

A stochastic gravitational-wave (GW) background consists of a large number of weak, independent and uncorrelated events of astrophysical or cosmological origin. The GW power on the sky is assumed to contain anisotropies on top of an isotropic component, i.e., the angular monopole. Complementary to the LIGO--Virgo--KAGRA (LVK) searches, we develop an efficient analysis pipeline to compute the maximum-likelihood anisotropic sky maps in stochastic backgrounds directly in the sky pixel domain using data folded over one sidereal day. We invert the full pixel-pixel correlation matrix in map-making of the GW sky, up to an optimal eigenmode cutoff decided systematically using simulations. In addition to modeled mapping, we implement a model-independent method to probe spectral shapes of stochastic backgrounds. Using data from LIGO--Virgo's first three observing runs, we obtain upper limits on anisotropies as well as the isotropic monopole as a limiting case, consistent with the LVK results. We also set constraints on the spectral shape of the stochastic background using this novel model-independent method.