The evolution of intermediate mass black holes in X-ray binaries

An ultra-luminous X-ray source, such as the one in M82, may very well be a signature of the presence of a black hole of intermediate mass. Such a black hole can be formed in a young and dense star cluster during the core collapse of the cluster, and subsequently capture a massive companion star. As the captured companion evolves it will overflow its Roche-lobe to start an episode of mass transfer. As long as the donor is still on the main-sequence, the mass transfer rate will be insufficient to power an ultra-luminous X-ray source, but as soon as it ascends the giant branch the X-ray luminosity increases by more than two orders of magnitude. Eventually, the companion to the intermediate mass black hole collapses to a black hole or a neutron star in a supernova explosion. In the latter case, the former X-ray source will become visible as a radio pulsar in orbit around an intermediate mass black hole. We predict that several of such sources will be detectable with the Low Frequency Array LOFAR.