title: Formation of a black hole in the dark.
abstract: 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 approximately 10-solar mass black hole of Cygnus X-1, the upper limit for the mass that could have been suddenly ejected is approximately 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.
authors.affiliation: Service d'Astrophysique-CEA-Saclay, 91191 Gif-sur-Yvette, France. fmirabel@cea.fr
authors.forename: I Félix
authors.initials: IF
authors.lastname: Mirabel
authors.forename: Irapuan
authors.initials: I
authors.lastname: Rodrigues