Most X-ray novae (aka soft X-ray transients) contain black hole primaries. In
particular, the large mass functions measured for six X-ray novae directly
clinch the argument (within general relativity) that they contain black holes.
These firm dynamical results are discussed, and the urgent need to determine
precise masses for black holes is stressed. The dynamical evidence for black
holes is convincing but it is indirect. Now it appears that direct evidence may
be at hand. Three recent studies have revealed phenomena that very likely probe
strong gravitational fields: (1) a comparison of the luminosities of black hole
systems and neutron star systems has yielded compelling evidence for the
existence of event horizons; (2) RXTE observations of fast, stable QPOs have
probed the very inner accretion disks of two black holes; and (3) three
different types of low energy spectra have been linked to different black-hole
spin states (e.g. Kerr vs. Schwarzschild).Comment: To appear in "Accretion Processes in Astrophysical Systems,"
Proceedings of the 8th Annual Astrophysics Conference in Maryland, eds. S.S.
Holt & T. Kallman (NY: AIP
