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 M_{BH, min}. This minimum mass depends primarily on the donor mass and evolutionary state. We show that M_{BH, 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.