Stellar microlensing of gravitational waves in the presence of strong lensing

Published in the Monthly Notices of Royal Astronomical Society.

Microlenses often lurk in the near-vicinity of galaxy lenses. The galaxy lens properties alter microlensing, causing the effective Einstein radius of small lenses near the trajectory of strongly lensed gravitational waves to grow. We investigated this interplay and demonstrated that gravitational waves lensed by a galaxy might offer a relatively clean environment to study strong lensing, free of contamination by stellar microlenses. This is particularly interesting for localization studies and the strong lensing science case, where stellar microlensing typically acts as a contaminant that introduces a sizeable variation in magnification measurements.

Like electromagnetic waves, gravitational waves can be gravitationally lensed by intervening objects, such as stars, black holes, galaxies, and galaxy clusters. However, while the theory behind the lensing of gravitational waves is similar to that of light lensing, the methods to detect it are entirely different due to fundamentally different sources and detectors. In particular, we may detect lensing magnification as an overall amplification of the waves. This magnification would cause binary merger signals to appear as coming from closer and higher-mass sources than they really do. Multiple images would appear as “repeated” events: near-identical events appearing minutes to months (or sometimes years) apart. As the lens will typically produce image separations that are far too tiny to be resolved with the current detectors, the events appear to come from the same sky location.

If smaller objects such as stars reside near the trajectory of the waves, microlensing occurs. The microlensing produces tiny lensing time delays, which can cause multiple lensed waveforms to overlap at the detectors and produce waveform “beating patterns.”

It is generally difficult to account for these beating patterns due to what we refer to as the wave optics effects. These effects are known to suppress microlensing (pointed out previously by, e.g., Oguri). This suppression limits our ability to detect microlenses in the low-mass limit. It is, in this sense, a hindrance for microlensing searches. On the other hand, the suppression can be seen as an advantage for the science case: If gravitational waves are unaffected by microlensing, they can offer a clean probe of strong lensing. 

Here we have studied the effect of microlensing on gravitational waves in the presence of strong lensing and discussed the scientific implications.

Link to paper