A Comparison of Intermediate-mass Black Hole Candidate Ultraluminous X-ray Sources 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 sources (ULXs) with ergs s 1 in nearby galaxies. These components can be well 40 L ≥ 10 X 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 black holes ( ergs s ), these sources are extremely promising 39 10 M L 1.3 # 10 , Edd 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 -kT X diagram, suggesting these ULXs may harbor intermediate-mass black holes. We briefly discuss the important strengths and weaknesses of this interpretation. Subject headings: accretion, accretion disks — black hole physics — galaxies: stellar content — relativity — X-rays: binaries