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

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Abstract
Current simulations of the rate at which stellar-mass compact objects merge with supermassive black holes (called extreme mass ratio inspirals, or EMRIs) focus on two-body capture by emission of gravitational radiation. The gravitational wave signal of such events will likely involve a significant eccentricity in the sensitivity range of the Laser Interferometer Space Antenna (LISA). We show that tidal separation of stellar-mass compact object binaries by supermassive black holes will instead produce events whose eccentricity is nearly zero in the LISA band. Compared to two-body capture events, tidal separations have a high cross section and result in orbits that have a large pericenter and small apocenter. Therefore, the rate of interactions per binary is high and the resulting inspirals are relatively unaffected by subsequent perturbations by other stars. Depending on the fraction of compact objects that are in binaries within a few parsecs of the center, the rate of low-eccentricity LISA events could be comparable to or larger than the rate of high-eccentricity events. This relative rate depends strongly on properties of compact binaries that are difficult to characterize using other observational techniques. Therefore, LISA observations of the eccentricity distribution will provide information, integrated over the galaxies with observed EMRIs, about compact binaries in galactic nuclei

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