Hydrodynamic Drag on a Compact Star Orbiting a Supermassive Black Hole

Abstract

The proposed Laser Interferometer Space Antenna is expected to detect gravitational waves from compact stars spiraling into supermassive black holes in distant galactic nuclei. Analysis of the inspiral events will require careful comparison of the observed signals with theoretical waveform templates. The comparison could be seriously compromised if nongravitational torques modify the orbit of the star. This paper estimates the torque exerted on an orbiting star as a result of hydrodynamic interactions with an accretion how around the supermassive black hole. It is argued that the majority of inspiral events will take place in low-luminosity galactic nuclei in which the mass accretion rate is low and the accretion occurs via an advection-dominated flow. The hydrodynamic torque is negligibly small in such systems and will have no effect on gravitational wave experiments.