Probing AGN Disks Density Profiles through Gravitational Wave Observations

Abstract

Massive black holes surrounded by a gaseous disk have been a prevailing model to explain a wide spectrum of astrophysical phenomena related to active galactic nucle (AGNs). However, direct and precise measurements of the disk density profiles remain elusive for current telescopes. In this work, we demonstrate that it is possible to pinpoint the gas density if an inspiralling stellar mass binary black hole is embedded in the AGN disk. Furthermore, if the barycenter of the pair follows an eccentric orbit around an AGN, then space-borne gravitational wave detectors can measure the density of the surrounding disk with multi-year observations by tracking the gravitational wave evolution. The error between the inferred density profile and the injected truth can be constrained to below $2\times10^{-11}\rm g/cm^3$. Our work opens up an exciting new channel to investigate the very center of galaxies, where disk gas density distributions $ρ(r)$ can be recovered by analyzing time-dependent environmental imprints in gravitational waveforms.

Figures (3)

• Figure 1: The posterior distribution of three parameters $\Delta M_c,\eta,\log\rho$ of a GW190521-like system. Black dashed lines represent the injected value, and light blue contour lines are 68% and 95% credible intervals (CI). And subplot headers report central estimates with $1\sigma$ uncertainties.
• Figure 2: The upper panel shows the injected (black line, SG model) and recovered (blue shaded region) disk density profile with an example event. Dots and error bars indicate the estimate of gas density and radial distance of the 3-month segments. The uncertainties are so small that we highlight the difference between the injection and the recovery in the zoomed-in panel. The lower panel shows the evolution of the gas density precision $\delta\rho$ for a population (grey) and the example (red) GW190521-like event.
• Figure 3: The estimate precision $\delta\rho$ across bbh systems with different total mass $M$ and mass ratio $q$ with the 3-month observation (solid lines). $\delta\rho$ equal to $10^{-12}/10^{-11}/10^{-10}\text{g/cm}^3$ are shown with blue/red/green lines, respectively. The yellow dot represents the estimated precision of the gas density considering a GW190521-like event in 3 months.