Probing Strong Gravitational Fields in X-ray Novae
  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). 
Jeffrey E. McClintock