Formation of a Black Hole in the Dark

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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 40solar 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.