Ultraluminous X–ray Sources in External Galaxies

We investigate models for the class of ultraluminous non–nuclear X–ray sources (ULXs) seen in a number of galaxies and probably associated with star–forming regions. Models where the X–ray emission is assumed to be isotropic run into several difficulties. In particular formation of sufficient numbers of the required ultramassive black–hole X–ray binaries is problematic, and the likely transient behaviour of the resulting systems is not in good accord with observation. The assumption of mild X–ray beaming suggests instead that ULXs may represent a shortlived but extremely common stage in the evolution of a wide class of X–ray binaries. The best candidate for this is the phase of thermal–timescale mass transfer inevitable in many intermediate and high–mass X–ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high–mass X–ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive black holes. Subject headings: accretion, accretion discs — binaries: close — X-rays: stars