Detecting Black Hole—Neutron Star Binaries with the Laser Interferometer Space Antenna

The detection of gravitational-waves emitted by double compact objects is starting to inform our understanding of the evolution of massive stars in binary systems. Black hole–neutron star (BH–NS) binaries are potentially observable with the planned space-based gravitational-wave detector LISA, which is sensitive to mHz frequencies. We estimate the number of BH–NS binaries that will be detected by LISA. Using BH–NSs generated from the rapid binary population synthesis code COMPAS, we calculate the signal-to-noise ratio for each binary in LISA. Over the course of a 4-year LISA mission we estimate between 0 and 5 detections and we find the probability that there is at least one detection is 48%, assuming a signal-to-noise ratio threshold of 7. Extending the mission to 10 years improves this estimate to between 0 and 7 detections and a 69% probability. We demonstrate that, in principle, a BH–NS could be detected from galaxies as far as away as the Andromeda Galaxy. We show that these BH–NS binaries can typically be approximated as stationary gravitational-wave sources on the timescale of the LISA mission. We discuss the effect of different metallicity, eccentricity and star formation rate assumptions. Last, we discuss possible ways to distinguish BH–NS binaries from the far more numerous double white dwarf background. We conclude that BH–NS binaries will be an exciting class of sources potentially detectable by the future LISA mission that will help in refining models of massive binary evolution.