  Ultraluminous X-ray sources (ULXs) with X-ray luminosities larger than the
Eddington luminosity of stellar mass objects may be powered by intermediate
mass black holes (IBHs) of masses Mbh~10^3Msun. If IBHs form in young dense
stellar clusters, they can be fed by Roche lobe overflow from a tidally
captured massive (Ms>10Msun) stellar companion. After the donor leaves the main
sequence it forms a compact remnant, which spirals in due to gravitational wave
(GW) emission. We show that space based detectors such as the Laser
Interferometer Space Antenna are likely to detect several of these sources. GW
sources stemming from this scenario have small eccentricities which give
distinct GW signals. Detection of such a GW signal will unambiguously prove the
existence of IBHs, and support the hypothesis that some ULXs are powered by
IBHs with captured companions.
