  We show that the black hole in the x-ray binary Cygnus X-1 was formed in situ
and did not receive an energetic trigger from a nearby supernova. The
progenitor of the black hole had an initial mass greater than 40 solar masses
and during the collapse to form the ~10 solar mass black hole of Cygnus X-1,
the upper limit for the mass that could have been suddenly ejected is ~1 solar
mass, much less than the mass ejected in a supernova. The observations suggest
that high-mass stellar black holes may form promptly, when massive stars
disappear silently.
