MG11 
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Black hole masses in ultraluminous X-ray sources

The nature and origin of ultraluminous X-ray sources (ULXs) will remain a mystery until we can determine the mass of the accreting black holes. It was suggested from X-ray spectral modelling that a cool thermal component from the accretion disk is robust evidence of intermediate-mass black holes (~ 1000 solar masses). Here I show that such interpretation is not solid: in fact, the X-ray spectra may show a "soft deficit" (smeared absorption) rather than a "soft excess" (disk emission). If so, ULXs may not require black hole masses much larger than about 100 times solar. Finally, I outline a possible scenario for the formation of accreting black holes in that mass range, arguing that they are a direct consequence of fast cloud collisions in a low metal abundance environment.