Binary Encounters with Supermassive Black Holes: Zero-Eccentricity LISA Events

Current simulations of the rate at which stellar-mass compact objects mergewith supermassive black holes (called extreme mass ratio inspirals, or EMRIs)focus on two-body capture by emission of gravitational radiation. Thegravitational wave signal of such events will likely involve a significanteccentricity in the sensitivity range of the Laser Interferometer Space Antenna(LISA). We show that tidal separation of stellar-mass compact object binariesby supermassive black holes will instead produce events whose eccentricity isnearly zero in the LISA band. Compared to two-body capture events, tidalseparations have a high cross section and result in orbits that have a largepericenter and small apocenter. Therefore, the rate of interactions per binaryis high and the resulting systems are very unlikely to be perturbed by otherstars into nearly radial plunges. Depending on the fraction of compact objectsthat are in binaries within a few parsecs of the center, the rate oflow-eccentricity LISA events could be comparable to or larger than the rate ofhigh-eccentricity events.Comment: Final accepted version: ApJ Letters 2005, 631, L11