The defining characteristic of a black hole is that it possesses an ev
ent horizon through which matter and energy can fall in but from which
nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binar
ies, appear to confirm this fundamental property of black holes. SXTs
that are thought to contain accreting black holes display a large vari
ation of luminosity between their bright and their faint states, while
SXTs with accreting neutron stars have a smaller variation. This diff
erence is predicted if the former stars have horizons, and the latter
have normal surfaces.