Wave Effects in Gravitational Lensing of Gravitational Waves from Chirping Binaries

Abstract

In the gravitational lensing of gravitational waves, the wave optics should be used instead of the geometrical optics when the wavelength $λ$ of the gravitational waves is longer than the Schwarzschild radius of the lens mass $M_L$. For the gravitational lensing of the chirp signals from the coalescence of the super massive black holes at the redshift $z_S\sim 1$ relevant to LISA, the wave effects become important for the lens mass smaller than $\sim 10^8 M_{\odot}$. For such cases, we compute how accurately we can extract the mass of the lens and the source position from the lensed signal. We consider two simple lens models: the point mass lens and the SIS (Singular Isothermal Sphere). We find that the lens mass and the source position can be determined within $\sim 0.1% [(S/N)/10^3]^{-1}$ for the lens mass larger than $10^8 M_{\odot}$ and $\gsim 10% [(S/N)/10^3]^{-1}$ for the lens mass smaller than $10^7 M_{\odot}$ due to the diffraction effect, where $(S/N)$ is the signal to noise ratio of the unlensed chirp signals. For the SIS model, if the source position is outside the Einstein radius, only a single image exists in the geometrical optics approximation so that the lens parameters can not be determined. While in the wave optics cases we find that the lens mass can be determined even for $M_L < 10^8 M_{\odot}$. For the point mass lens, one can extract the lens parameters even if the source position is far outside the Einstein radius. As a result, the lensing cross section is an order of magnitude larger than that for the usual strong lensing of light.