It seems that it happens when they merge, since: > The cataclysmic event saw the black holes, one eight times more massive than the sun, the other 14 times mor

It seems that it happens when they merge, since: > The cataclysmic event saw the black holes, one eight times more massive than the sun, the other 14 times more massive, merge into one about 21 times heavier than the sun. In the process, energy equivalent to the mass of the sun radiated into space as gravitational waves. So the final black hole had rather less mass than the sum of the original two. https://www.theguardian.com/science/2016/jun/15/gravitational-waves-detected-from-collision-of-second-set-of-black-holes-ligo https://www.theguardian.com/science/2016/jun/15/gravitationa...