2011ApJ...730..140B

We investigate the future evolution of two extragalactic X-ray binaries: IC10 X-1 and NGC300 X-1. Each consists of a high-mass black hole (BH; ∼20-30 M_☉) accreting from a massive Wolf-Rayet (W-R) star companion ( ≳ 20 M_☉), and both are located in low-metallicity galaxies. We analyze the current state of the systems and demonstrate that both systems will very quickly ( ≲ 0.3 Myr) form close BH-BH binaries with short coalescence time (∼3 Gyr) and large chirp mass (∼15 M_☉). The formation of a BH-BH system seems unavoidable, as (1) W-R companions are well within their Roche lobes and do not expand, so no Roche lobe overflow is expected; (2) even intense W-R wind mass loss does not remove sufficient mass to prohibit the formation of the second BH; and (3) even if the BH receives a large natal kick, the systems are very closely bound and are almost impossible to disrupt. As there are two such immediate BH-BH progenitor systems within 2 Mpc, and as the current gravitational-wave instruments LIGO/VIRGO (initial stage) can detect such massive BH-BH mergers out to ∼200 Mpc, the empirically estimated detection rate of such inspirals is R = 3.36^+8.29_–2.92 at the 99% confidence level. If there is no detection in the current LIGO/VIRGO data (unreleased year of s6 run), the existence of these two massive BH systems poses an interesting challenge. Either the gravitational radiation search is not sensitive to massive inspirals, or there is some fundamental misunderstanding of stellar evolution physics leading directly to the formation of BH-BH binaries.