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