Wave-optical Effects in the Microlensing of Continuous Gravitational Waves by Star Clusters

Abstract

Rapidly rotating neutron stars are promising sources for existing and upcoming gravitational-wave interferometers. While relatively dim, these systems are expected to emit continuously, allowing for signal to be accumulated through persistent monitoring over year-long timescales. If, at some point during the observational window, the source comes to lie behind a dense collection of stars, transient gravitational lensing may occur. Such events, though rare, would modulate the waveform, induce phase drifts, and ultimately affect parameter inferences concerning the nuclear equation of state and/or magnetic field structure of the neutron star. Importantly, the radiation wavelength will typically exceed the Schwarzschild radius of the individual perturbers in this scenario, implying that (micro)lensing occurs in the diffractive regime, where geometric optics does not apply. In this paper, we make use of numerical tools that borrow from Picard–Lefschetz theory to efficiently evaluate the relevant Fresnel–Kirchhoff integrals for n ≳ 102 microlenses. Modulated strain profiles are constructed both in general and for particular neutron star trajectories relative to some simulated macrolenses.

Prospects for the Observation of Continuous Gravitational Waves from Spinning Neutron Stars Lensed by the Galactic Supermassive Black Hole

“…Such an observation would be a unique new way of measuring the mass of the galactic SMBH. In addition, compact objects and stars in the galactic center could produce additional microlensing effects on the GW signal, which are potentially measurable (Liao et al 2019;Marchant et al 2020;Suvorov 2022). This would be a powerful means of probing the astrophysical environment of the galactic center.…”

“…Such an observation might also enable us to constrain the presence of additional hairs of the black hole, thus probing the true nature of the supermassive compact object at the galactic center. In addition, stars and stellar-mass compact objects in the galactic center can cause additional microlensing effects on the CGW signal (e.g., Liao et al 2019;Suvorov 2022). This will potentially allow us to probe the poorly understood astrophysical environment of the galactic center.…”