On the other hand, binary x-ray sources exist and the black holes in them are
thought to be quitemassive (¨Ozel et al, 2010;Wiktorowicz, Belczynski, & Maccarone,
2014). Stars above 35M⊙ eithermake black holes if their mass loss during theWolf-
Rayet stage is small, or some variant of Type Ibc supernovae if it is large and shrinks
the carbon oxygen core below about 6 M⊙.