What happens when black holes collide? | EXPERT ANSWERS PHYSICS GIRL QUESTIONS

The black hole merger emitted 3 solar masses worth of gravitational waves. What would it be like in the vicinity of this merger (a few AU away, for example)? How close do you have to be before you have to worry about the dangers of gravitational radiation (if any)? Presumably, at that distance there'd be _some_ optical counterpart from dust and nearby debris.