Luckily, black holes are rarely formed in complete isolation. There is almost always other matter around. For example, binary star systems, which contain two stars in close orbit, are very common. This slide is an artist's rendition of what it would look like of one of the stars in a binary system collapsed into a black hole The intense gravitational field of the black hole sucks matter off of the companion star. The matter does not fall directly into the black hole. It swirls around and spirals in, much like water down a bathtub drain. As this matter fell towards the black hole it gains energy, and heats up to the point where it emits a great deal of radiation (x-rays in fact). This radiation is emitted while the matter is still relatively far from the black hole, so it can escape and this is what we detect. The evidence is somewhat circumstantial, since the same sorts of x-rays would be emitted even if the collapsed star was some other compact object such as a neutron star or white dwarf. However, if we can measure the masses of the two stars, and the collapsed star is heavy enough, theoretical arguments force us conclude that the x-rays are being emitted by matter falling in to a black hole.