Why all of the observed stellar -remmant black-hole candidates members of close binary systems?

Answer 1

Those are easier to observe than an isolated black hole, because the black holes are usually discovered because of infalling matter that emits X-rays, or because other objects move around an invisible object (the black hole).

Answer 2

I doubt that that is the case. For example, Sagittarius A* is not really part of a close binary system. But for the case of stellar black holes, members of close binary systems are easier to observe than black holes that are alone by themselves, or where the companion star is far away.

Answer 3

Stellar-remnant black holes in close binary systems are more easily detected because the material being pulled from the companion star forms an accretion disk around the black hole, emitting X-rays and making them observable. Additionally, the gravitational interactions in close binary systems can lead to the formation of black holes through stellar evolution processes. This makes it more likely for black holes to be found in such systems.

Answer 4

Those are easier to observe.