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J. Liu, J.N. Bregman (University of Michigan)
The ultraluminous X-ray sources (ULX) in external galaxies radiate at the Eddington luminosity of a 30-100 Msolar, so they could be “intermediate mass” black holes (103-104 Msolar) that are sub-Eddington emitters, or black holes of known mass (10 Msolar) that are super-Eddington emitters. To help decide between these alternatives, we have examined their location within their host galaxies and their variability, for the nearby galaxies (d < 10 Mpc) that have ULX sources. We find that nearly all of the ULX sources lie in star forming regions, such as spiral arms or starburst regions. Their proximity to such regions indicates that they are most likely associated with massive stellar systems and that the lifetimes of these ULXs are relatively brief (<3E7 yr). The accretion rate for these systems naturally implies the presence of a companion star that is overflowing its Roche lobe, and as it may be difficult for a 103-104 Msolar black hole to acquire such a star, a 10 Msolar black hole appears to be more likely as the primary.