THE fate of Cyg X-1: An empirical lower limit on black-hole-neutron-star merger rate

Abstract

The recent distance determination allowed precise estimation of the orbital parameters of Cyg X-1, which contains a massive 14.8 M ⊙ black hole (BH) with a 19.2 M ⊙ O star companion. This system appears to be the clearest example of a potential progenitor of a black hole + neutron star (BH-NS) system. We follow the future evolution of Cyg X-1, and show that it will soon encounter a Roche lobe overflow episode, followed shortly by a Type Ib/c supernova and the formation of a neutron star (NS). It is demonstrated that in majority of cases (≳ 70%) the supernova and associated natal kick disrupt the binary due to the fact that the orbit expanded significantly in the Roche lobe overflow episode. In the reminder of cases (≲ 30%) the newly formed BH-NS system is too wide to coalesce in the Hubble time. Only sporadically (∼1%) may a Cyg X-1-like binary form a coalescing BH-NS system given a favorable direction and magnitude of the natal kick. If a Cyg X-1-like channel (comparable mass BH-O star bright X-ray binary) is the only or dominant way to form BH-NS binaries in the Galaxy, then we can estimate the empirical BH-NS merger rate in the Galaxy at the level of ∼ 0.001Myr-1. This rate is so low that the detection of BH-NS systems in gravitational radiation is highly unlikely, generating Advanced LIGO/VIRGO detection rates at the level of only ∼ 1 per century. If BH-NS inspirals are in fact detected, it will indicate that the formation of these systems proceeds via some alternative and yet unobserved channels. © 2011. The American Astronomical Society. All rights reserved.