A Sea of Black Holes: Characterizing the LISA Signature for Stellar-Origin Black Hole Binaries

Observations by the LIGO, Virgo and KAGRA (LVK) detectors have provided new insights in the demographics of stellar-origin black hole binaries (sBHB). A few years before gravitational-wave signals from sBHB mergers are recorded in the LVK detectors, their early coalescence will leave a unique signature in the ESA/NASA mission Laser Interferometer Space Antenna (LISA). Multiband observations of sBHB sources between LISA and LVK detectors opens an unprecedented opportunity to investigate the astrophysical environment and multi-messenger early-alerts. In this study, we report the sBHB sources that will be present in the LISA data derived directly from the hydrodynamic cosmological simulation Illustris. By surveying snapshots across cosmological volume, metallicity and look-back time, we find that about tens to thousand sBHB candidates will be present in the LISA data for various combinations of mission lifetime. For estimates consistent with the LVK rates, we find that only 20 sBHBs across Illustris snapshots will be detected with significant confidence for a 10-year LISA mission, while a 4-year LISA mission would detect only 2 sBHBs. Our work paves the way for creating LISA mock data and bench marking LISA detection pipelines directly using cosmological simulations.