The nearby NS grows in mass by accreting matter from the supernova ejecta. Soon, the NS becomes too massive and collapses gravitationally to form a black hole (BH) with at least ~2.5 times the Sun’s mass. An energetic gamma ray burst (GRB) is produced during the collapse. The tightly-bound νNS- BH binary system continues to evolve into an ever tighter configuration via the emission of gravitational waves. Such a νNS- BH binary system may be detectable from its gravitational wave signal. Within a span of 10,000 years or less, the νNS-BH binary eventually merge, driving an ultra-short duration GRB. Two supernovae (i.e. plural of supernova) and two GRBs later, a binary system of two massive stars is now a single black hole drifting through space.
C. L. Fryer, F. G. Oliveira, J. A. Rueda, R. Ruffini (2015), “On the Neutron Star-Black Hole Binaries Produced by Binary-driven Hypernovae”, arXiv:1505.02809 [astro-ph.HE]