Fuzzy dark matter soliton as gravitational lens

Abstract: The Schr\"odinger-Poisson (SP) equations predict fuzzy dark matter (FDM) solitons. Given the FDM mass $\sim10^{-20}\rm~{eV}/c^2$, the FDM soliton in the Milky Way is massive $\sim 10^7~M_{\odot}$ but diffuse $\sim 10{\rm~pc}$. Therefore, this FDM soliton can serve as a gravitational lens for gravitational waves (GWs) with frequency $\sim10^{-8}{\rm~Hz}$. In this paper, we investigate its gravitational lensing effects by numerical simulation of the propagation of GWs through it. We find that the maximum magnification factor of GWs is very small $\sim10^{-4}$, but the corresponding magnification zone is huge $\sim6{\rm~pc}$ for FDM with mass equal to $8^{-21}\rm~{eV}/c^2$. Consequently, this small magnification factor in that large magnification zone means a small antisotropy of $\sim10^{-4}$ over a large solid angle in the GW background, even though this antisotropy is out of the sensitivity of the pulsar timing arrays today.