Preparing LISA for Intermediate-Mass Black Hole Science

The LIGO/Virgo gravitational-wave (GW) detectors have already demonstrated the capacity of GW astronomy to yield important and unexpected discoveries. More than ten merging black hole (BH) binaries have been detected, some with masses considerably larger than had been anticipated, such as the recent event GW190521. Stellar-mass and intermediate-mass BH binaries should be detectable by the space-based GW observatory LISA hours to years before those binaries become visible to ground-based GW detectors. At the same time, one can exploit the ground-based binary parameter measurements to greatly focus the search through archived LISA data, and hence reduce the signal-to-noise threshold needed to confidently identify the binary signals in the low-frequency data set. The synergies that result when the same binaries are observed by instruments in space and on the ground will enable novel ways to exploit the data. Multiband observations will yield complementary information that can remove degeneracies between parameters and improve our understanding of the BH binary population. Observations of the population distribution across the GW spectrum could provide strong constraints on BH binary formation channels, through measurements of spin evolution and eccentricity, and provide a unique opportunity to test General Relativity. Multiband GW physics can potentially provide an unprecedented amount of precise data, beyond what is achievable from either space-based or ground-based GW data alone.