  Although General Relativity had provided the physical basis of black holes,
evidence for their existence had to await the Space Era when X-ray observations
first directed the attention of astronomers to the unusual binary stars Cygnus
X-1 and A0620-00. Subsequently, a number of faint Ariel 5 and Uhuru sources,
mainly at high Galactic latitude, were found to lie close to bright Seyfert
galaxies,suggesting the nuclear activity in AGN might also be driven by
accretion in the strong gravity of a black hole. Detection of rapid X-ray
variability with EXOSAT later confirmed that the accreting object in AGN was
almost certainly a supermassive black hole.
