An Observational Diagnostic for Ultraluminous X-Ray Sources

We consider observational tests for the nature of ultraluminous X-ray (ULX) sources. These tests must distinguish between thermal-timescale mass transfer onto stellar-mass black holes, leading to anisotropic X-ray emission, and accretion onto intermediate-mass black holes. We suggest that long-term transient behavior via the thermal-viscous disk instability could discriminate between these two possibilities for ULX sources in regions of young stellar populations. Thermal-timescale mass transfer generally produces stable disks and persistent X-ray emission. In contrast, mass transfer from massive stars to black holes produces unstable disks and thus transient behavior, provided that the black hole mass exceeds some minimum value MBH, min. This minimum mass depends primarily on the donor mass and evolutionary state. We show that MBH, min ≳ 50 M☉ for a large fraction (≳90%) of the mass transfer lifetime for the most likely donors in young clusters. Thus, if long-term monitoring reveals a large transient fraction among ULX sources in a young stellar population, these systems would be good candidates for intermediate-mass black holes in a statistical sense; information about the donor star is needed to make this identification secure in any individual case. A transient ULX population would imply a much larger population of quiescent systems of the same type.