Gravity Wave Sources
The two main categories of gravitational-wave sources for LISA are the galactic binaries and the massive black holes (MBHs) expected to exist in the centres of most galaxies.
Because the masses involved in typical binary star systems are small (a few solar masses), the observation of binaries is limited to our Galaxy. Galactic sources that can be detected by LISA include a wide variety of binaries, such as pairs of close white dwarfs, pairs of neutron stars, neutron star and black hole (5 - 20 solar masses) binaries, pairs of contacting normal stars, normal star and white dwarf (cataclysmic) binaries, and possibly also pairs of black holes. It is likely that there are so many white-dwarf binaries in our Galaxy that they cannot be resolved at frequencies below 10-3 Hz, leading to a confusion-limited background. Some galactic binaries are so well studied, especially the X-ray binary 4U1820-30, that it is one of the most reliable sources. If LISA would not detect the gravitational waves from known binaries with the intensity and polarisation predicted by General Relativity, it would shake the very foundations of gravitational physics.
The main objective of the LISA mission, however, is to learn about the formation, growth, space density and surroundings of massive black holes (MBHs). There is now compelling indirect evidence for the existence of MBHs with masses of 106 to 108 solar masses in the centres of most galaxies, including our own. The most powerful sources are the mergers of MBHs in distant galaxies, with amplitude signal-to-noise ratios of several thousands for 106 solar mass black holes. Observations of signals from these sources would test General Relativity, and particularly black-hole theory, to unprecedented accuracy. Not much is currently known about black holes with masses ranging from about 100 solar masses to 106 solar masses. LISA can provide unique new information throughout this mass range.
(Extract from LISA: Detecting and observing gravitational waves, the Mission Summary of the Cornerstone Study Results, ESA brochure 164.)