The influence of the hydrodynamic drag from an accretion torus on extreme mass-ratio inspirals

TL;DR

This study investigates how hydrodynamic drag from a torus affects extreme mass-ratio inspirals around black holes, finding it generally small but potentially detectable and characteristic in certain astrophysical configurations with implications for gravitational wave observations.

Contribution

It provides the first detailed analysis of hydrodynamic drag effects on EMRIs in spacetimes with a rotating black hole and a non self-gravitating torus, highlighting conditions where this effect is significant.

Findings

Hydrodynamic drag is usually smaller than radiation reaction in EMRIs.

In certain configurations, hydrodynamic drag can be comparable to radiation reaction.

Hydrodynamic drag can alter the orbital inclination in detectable ways.

Abstract

We have studied extreme mass-ratio inspirals (EMRIs) in spacetimes containing a rotating black hole and a non self-gravitating torus with constant specific angular momentum. We have found that the effect of the hydrodynamic drag exerted by the torus on the satellite is much smaller than the corresponding one due to radiation reaction, for systems such as those generically expected in AGNs and at distances from the SMBH which can be probed with LISA. However, given the uncertainty on the parameters of these systems, there exist configurations in which the effect of the hydrodynamic drag can be comparable to the radiation-reaction one in phases of the inspiral which are detectable by LISA. This is the case, for instance, for a 10^6 M_sun SMBH surrounded by a corotating torus of comparable mass and with radius of 10^3-10^4 gravitational radii, or for a 10^5 M_sun SMBH surrounded by a corotating 10^4 M_sun torus with radius of 10^5 gravitational radii. Should these conditions be met in astrophysical systems, EMRI-gravitational waves could provide a characteristic signature of the presence of the torus. In fact, while radiation reaction always increases the inclination of the orbit with respect to the equatorial plane, the hydrodynamic drag from a torus corotating with the SMBH always decreases it. However, even when initially dominating over radiation reaction, the influence of the hydrodynamic drag decays very rapidly as the satellite moves into the very strong-field region of the SMBH (i.e., p <~ 5M). Although our results have been obtained for a specific class of tori, we argue that they will be qualitatively valid also for more generic distributions of the specific angular momentum.