Observational evidence for stellar mass binary black holes and their coalescence rate

We review the formation scenarios for binary black holes, and show that their coalescence rate depends very strongly on the outcome of the second mass transfer. However, the observations of IC10 X-1, an binary with a massive black hole accreting from a Wolf-Rayet star proves that this mass transfer can be stable. We analyze the future evolution of IC10 X-1 and show that it is very likely to form a binary black hole system merging in a few Gyrs. We estimate the coalescence rate density of such systems to be $ 0.06 {\rm Mpc}^{-3} {\rm Myr}^{-1}$,andthedetectionrateforthecurrentLIGO\mathrm{/}VIRGOof$ 0.69 {\rm yr}^{-1} $, a much higher value than the expected double neutron star rate. Thus the first detection of a gravitational wave source is likely to be a coalescence of a binary black hole.