Journal
ASTROPHYSICAL JOURNAL
Volume 614, Issue 2, Pages L117-L120Publisher
IOP PUBLISHING LTD
DOI: 10.1086/425316
Keywords
accretion, accretion disks; black hole physics; galaxies : stellar content; relativity; X-rays : binaries
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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 greater than or equal to 10(49) ergs s(-1) in nearby galaxies. These components can be well 40 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-circle dot black holes (L-Edd similar or equal to 1.3 x 10(39) ergs s(-1)), 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.
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