Detecting the Beaming Effect of Gravitational Waves

TL;DR

This paper explores the beaming effect of gravitational waves caused by relative motion between source and observer, revealing that it affects amplitude and polarization, potentially impacting GW detection accuracy.

Contribution

It introduces the concept of a beaming effect for gravitational waves and analyzes its impact on observed signals, which was not considered in prior models.

Findings

Observed GW amplitude varies non-monotonically with velocity.

Even small velocities cause systematic errors exceeding calibration accuracy.

Ignoring the effect can lead to incorrect source parameter estimation.

Abstract

The models currently used in the detection of gravitational waves (GWs) either do not consider a relative motion between the center-of-mass of the source and the observer, or usually only consider its effect on the frequencies of GWs. However, it is known for light waves that a relative motion not only changes the frequencies but also the brightness of the source, the latter of which is called the `beaming effect'. Here we investigate such an effect for GWs and find that the observed amplitude of a GW signal, unlike the behavior of light, is not a monotonic function of the relative velocity and responds differently to the two GW polarizations. We attribute the difference to a rotation of the wave-vector, as well as a reorientation of the GW polarizations. We find that even for velocities as small as $0.25\%$ of the speed of light, ignoring the aforementioned beaming effect could induce a systematic error that is larger than the designated calibration accuracy of LIGO. This error could lead to an incorrect estimation of the distance and orbital inclination of a GW source, or result in a spurious signal that appears to be incompatible with general relativity.