Angular resolution of a Bayesian search for anisotropic stochastic gravitational wave backgrounds with LISA

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 l_max ¼ 16; this exceeds the maximum angular resolution achieved by other map-making techniques for LISA ASGWBs.