A Comparison of Intermediate Mass Black Hole Candidate Ulxs and Stellar-mass Black Holes

Cool thermal emission components have recently been revealed in the X-ray spectra of a small number of ultraluminous X-ray (ULX) sources with LX ≥ 1040 erg/s in nearby galaxies. These components can be well fitted with accretion disk models, with temperatures approximately 5–10 times lower than disk temperatures measured in stellar-mass Galactic black holes when observed in their brightest states. Because disk temperature is expected to fall with increasing black hole mass, and because the X-ray luminosity of these sources exceeds the Eddington limit for 10 M⊙ black holes (LEdd. ≃ 1.3× 1039 erg/s), these sources are extremely promising intermediate– mass black hole candidates (IMBHCs). In this Letter, we directly compare the inferred disk temperatures and luminosities of these ULXs, with the disk temperatures and luminosities of a number of Galactic black holes. The sample of stellar-mass black holes was selected to include different orbital periods, companion types, inclinations, and column densities. These ULXs and stellar-mass black holes occupy distinct regions of a LX – kT diagram, suggesting these ULXs may harbor IMBHs. We briefly discuss the important strengths and weaknesses of this interpretation.