Cyg X-3: A Galactic Double Black Hole or Black-Hole-Neutron-Star Progenitor

There are no known stellar-origin double black hole (BH–BH) or black-hole–neutron-star (BH–NS) systems. We argue that Cyg X-3 is a very likely BH–BH or BH-NS progenitor. This Galactic X-ray binary consists of a compact object, wind-fed by a Wolf-Rayet (W-R) type companion. Based on a comprehensive analysis of observational data, it was recently argued that Cyg X-3 harbors a 2–4.5 M ☉ black hole (BH) and a 7.5–14.2 M ☉ W-R companion. We find that the fate of such a binary leads to the prompt (≲ 1 Myr) formation of a close BH–BH system for the high end of the allowed W-R mass ( M W-R ≳ 13 M ☉ ). For the low- to mid-mass range of the W-R star ( M W-R ∼ 7–10 M ☉ ) Cyg X-3 is most likely (probability 70%) disrupted when W-R ends up as a supernova. However, with smaller probability, it may form a wide (15%) or a close (15%) BH–NS system. The advanced LIGO/VIRGO detection rate for mergers of BH–BH systems from the Cyg X-3 formation channel is ∼10 yr −1 , while it drops down to ∼0.1 yr −1 for BH–NS systems. If Cyg X-3 in fact hosts a low-mass black hole and massive W-R star, it lends additional support for the existence of BH–BH/BH–NS systems.