Item

Abstract

Gravitational-wave observations of extreme mass ratio inspirals (EMRIs) offer
the opportunity to probe the environments of active galactic nuclei (AGN)
through the torques that accretion disks induce on the binary. Within a
Bayesian framework, we study how well such environmental effects can be
measured using gravitational wave observations from the Laser Interferometer
Space Antenna (LISA). We focus on the torque induced by planetary-type
migration on quasicircular inspirals, and use different prescriptions for
geometrically thin and radiatively efficient disks. We find that LISA could
detect migration for a wide range of disk viscosities and accretion rates, for
both $\alpha$ and $\beta$ disk prescriptions. For a typical EMRI with masses
$50M_\odot+10^6M_\odot$, we find that LISA could distinguish between migration
in $\alpha$ and $\beta$ disks and measure the torque amplitude with $\sim 20\%$
relative precision. Provided an accurate torque model, we also show how to turn
gravitational-wave measurements of the torque into constraints on the disk
properties. Furthermore, we show that, if an electromagnetic counterpart is
identified, the multimessenger observations of the AGN EMRI system will yield
direct measurements of the disk viscosity. Finally, we investigate the impact
of neglecting environmental effects in the analysis of the gravitational-wave
signal, finding 3$\sigma$ biases in the primary mass and spin, and showing that
ignoring such effects can lead to false detection of a deviation from general
relativity. This work demonstrates the scientific potential of gravitational
observations as probes of accretion-disk physics, accessible so far through
electromagnetic observations only.