Evolution of gas disc-embedded intermediate mass ratio inspirals in the LISA band

TL;DR

This study uses hydrodynamical simulations to analyze how gas torques in AGN discs influence intermediate-mass ratio inspirals, affecting gravitational wave signals detectable by LISA, and explores the potential to constrain disc properties.

Contribution

It provides a systematic analysis of disc torques on IMRIs in AGN environments, highlighting their impact on GW waveforms and potential for constraining disc physics.

Findings

Gas torques are comparable to planetary migration torques but vary with parameters.

LISA can detect gas-induced waveform deviations for dense AGN discs.

Higher disc viscosity amplifies torque effects and waveform deviations.

Abstract

Among the potential milliHz gravitational wave (GW) sources for the upcoming space-based interferometer LISA are extreme- or intermediate-mass ratio inspirals (EMRI/IMRIs). These events involve the coalescence of supermassive black holes in the mass range $10^5 M_{\odot} \lesssim M \lesssim 10^7 M_{\odot}$ with companion BHs of much lower masses. A subset of E/IMRIs are expected to occur in the accretion discs of active galactic nuclei (AGN), where torques exerted by the disc can interfere with the inspiral and cause a phase shift in the GW waveform. Here we use a suite of two-dimensional hydrodynamical simulations with the moving-mesh code DISCO to present a systematic study of disc torques. We measure torques on an inspiraling BH and compute the corresponding waveform deviations as a function of the binary mass ratio $q\equiv M_2/M_1$, the disc viscosity ($\alpha$), and gas temperature (or equivalently Mach number; $\mathcal{M}$). We find that the absolute value of the gas torques is within an order of magnitude of previously determined planetary migration torques, but their precise value and sign depends non-trivially on the combination of these parameters. The gas imprint is detectable by LISA for binaries embedded in AGN discs with surface densities above $\Sigma_0\ge10^{4-6} \rm \, g cm^{-2}$, depending on $q$, $\alpha$ and $\mathcal{M}$. Deviations are most pronounced in discs with higher viscosities, and for E/IMRIs detected at frequencies where LISA is most sensitive. Torques in colder discs exhibit a noticeable dependence on the GW-driven inspiral rate as well as strong fluctuations at late stages of the inspiral. Our results further suggest that LISA may be able to place constraints on AGN disc parameters and the physics of disc-satellite interaction.