Interference patterns for simple lens models in wave-optics regime

TL;DR

This paper investigates wave optics interference patterns in simple lens models relevant to gravitational wave lensing, highlighting differences from geometric optics near caustics and implications for lens identification.

Contribution

It provides a detailed analysis of interference patterns in wave optics for point mass, Chang-Refsdal, and binary lenses in the gravitational wave frequency band, revealing new features near caustics.

Findings

Interference oscillations differ significantly near caustics in wave vs. geometric optics.

Wave optics convergence to geometric optics away from caustics, but with amplitude differences.

Massive lenses (~100-200 M_sun) can cause notable de-amplification at 100 Hz near caustics.

Abstract

This work studies interference patterns created by simple lens models (point mass, Chang-Refsdal, and binary lens) in the wave optics regime, primarily in the context of lensing of gravitational waves (GWs) in the LIGO band at frequencies around 100 Hz. We study how the interference patterns behave close to the caustic curves which mark the high magnification regions in conventional geometric optics. In addition, we also look at the formation of highly de-amplified regions in the amplification maps close to caustics and how they differ under wave and geometric optics. We see that for a source close to caustics, the oscillations in the amplification factor (their amplitude and location of crests and troughs) can differ significantly in wave optics compared to geometric optics. As we move away from caustics, the wave optics amplification factor starts to converge towards geometric optics one, especially the frequencies at which crests and through occur in the amplification factor, although the amplitude of these oscillations can still be considerably different. For Chang-Refsdal and binary lens with ${\sim}100\:{\rm M_\odot}-200\:{\rm M_\odot}$ can introduce significant de-amplification at frequencies ${\sim}100$ Hz when the source is close to caustics, which may help us distinguish such lenses from the point mass lens.