http://news.sciencemag.org/scienceno...s-massive.html

"... the black-hole's progenitor was actually the more massive of the
two, tipping the scales at 97 solar masses. But the big star fused its
hydrogen and helium into heavier elements with lightning speed,
expelling gigantic solar winds. The companion star lapped up gobs of
that expelled material, growing quickly to its current size. When the
progenitor star had no more helium to fuse, it collapsed into a black
hole—which we now see is pulling matter back in from the companion star
at a furious clip".