Can Einstein (rings) surf Gravitational Waves?

TL;DR

This paper investigates how gravitational waves from a supermassive black hole binary affect Einstein rings, finding that while size changes are undetectable, induced time delays could potentially allow Einstein rings to serve as gravitational wave detectors.

Contribution

It introduces a novel analysis of gravitational wave effects on Einstein rings, highlighting the potential for using them as gravitational wave detectors through induced time delays.

Findings

Size changes of Einstein rings are too small to detect with current technology.

Orthogonal gravitational waves induce measurable time delays in photon paths.

Detection of such time delays could enable Einstein rings to act as gravitational wave detectors.

Abstract

How does the appearance of a strongly lensed system change if a gravitational wave is produced by the lens? In this work we address this question by considering a supermassive black hole binary at the center of the lens emitting gravitational waves propagating either colinearly or orthogonally to the line of sight. Specializing to an Einstein ring configuration (where the source, the lens and the observer are aligned), we show that the gravitational wave induces changes on the ring's angular size and on the optical path of photons. The changes are the same for a given pair of antipodal points on the ring, but maximally different for any pair separated by $90^{\circ}$. For realistic lenses and binaries, we find that the change in the angular size of the Einstein ring is dozens of orders of magnitude smaller than the precision of current experiments. On the other hand, the difference in the optical path induced on a photon by a gravitational wave propagating \textit{orthogonally} to the line of sight triggers, at peak strain, time delays in the range $\sim 0.01 - 1$ seconds, making the chance of their detection (and thus the use of Einstein rings as gravitational wave detectors) less hopeless.