2004ApJ...614L.117M

Cool thermal emission components have recently been revealed in the X-ray spectra of a small number of ultraluminous X-ray sources (ULXs) with L_X≥10⁴⁰ ergs/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 (L_Edd≃1.3x10³⁹ ergs/s), these sources are extremely promising intermediate-mass black hole candidates. 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 L_X-kT diagram, suggesting these ULXs may harbor intermediate-mass black holes. We briefly discuss the important strengths and weaknesses of this interpretation.