Formation and evolution of intermediate mass black hole X-ray binaries

The evolution of young (10 Myr) star clusters with a density exceeding about 10⁵ star pc^-3 are strongly affected by physical stellar collisions during their early lifetime. In such environments the same star may participate in several tens to hundreds of collisions ultimately leading to the collapse of the star to a black hole of intermediate mass. At later time, the black hole may acquire a companion star by tidal capture or by dynamical - three-body - capture. When the captured star evolves it starts to fill its Roche-lobe and transfers mass to its accompanying black hole. This then leads to a bright phase of X-ray emission, which lasts for the remaining main-sequence lifetime of the donor. If the star captured by the intermediate mass black hole is relatively low mass (2 M_·) the binary will also be visible as a bright source in gravitational waves. Based on empirical models we argue that, for as long as the donor remains on the main sequence, the source will be ultraluminous L _x 10⁴⁰erg s^-1 for about a week every few month. When the donor star is more massive 15 M _· or evolved off the main sequence the bright time is longer, but the total accretion phase lasts much shorter.