Angular Resolution of a Bayesian Search for Anisotropic Stochastic Gravitational Wave Backgrounds with LISA

TL;DR

This paper assesses LISA's ability to resolve anisotropic stochastic gravitational wave backgrounds using Bayesian spherical harmonics, demonstrating that resolution improves with more coefficients until computational limits are reached at 6.

Contribution

It introduces a Bayesian spherical harmonic framework for LISA to characterize anisotropic GW backgrounds and evaluates its resolution limits.

Findings

Resolution improves with more spherical harmonic coefficients.

Maximum resolution limit identified at 6 coefficients due to computational constraints.

LISA's resolution surpasses previous map-making techniques.

Abstract

The Laser Interferometer Space Antenna (LISA), a spaceborne gravitational wave (GW) detector set to launch in 2035, will observe several stochastic GW backgrounds in the mHz frequency band. At least one of these signals -- arising from the tens of millions of unresolved white dwarf binaries in the Milky Way -- is expected to be highly anisotropic on the sky. We evaluate the angular resolution of LISA and its ability to characterize anisotropic stochastic GW backgrounds (ASGWBs) using the Bayesian Spherical Harmonic formalism in the Bayesian LISA Inference Package (BLIP). We use \blip to simulate and analyze ASGWB signals in LISA across a large grid in total observing time, ASGWB amplitude, and angular size. We consider the ability of the \blip anisotropic search algorithm to both characterize single point sources and to separate two point sources on the sky, using a full-width half-max (FWHM) metric to measure the quality and spread of the recovered spatial distributions. We find that the number of spherical harmonic coefficients used in the anisotropic search model is the primary factor that limits the search's angular resolution. Notably, this trend continues until computational limitations become relevant around $\ell_{\mathrm{max}}=16$; this exceeds the maximum angular resolution achieved by other map-making techniques for LISA ASGWBs.