  If a black hole has a low spin value, it must double its mass to reach a high
spin parameter. Although this is easily accomplished through mergers or
accretion in the case of supermassive black holes in galactic centers, it is
impossible for stellar-mass black holes in X-ray binaries. Thus, the spin
distribution of stellar-mass black holes is almost pristine, largely reflective
of the angular momentum imparted at the time of their creation. This fact can
help provide insights on two fundamental questions: What is the nature of the
central engine in supernovae and gamma-ray bursts? and What was the spin
distribution of the first black holes in the universe?
