CygX-1/A candidate of the black hole

Among discrete galactic X-ray sources, Cyg X-1 has been noted for its peculiar features in several respects. It is one of the few sources with a hard power law spectrum extending beyond several hundred keV. Cyg X-1 also distinguishes itself by its profound time variability over a wide range of time scales. The most remarkable incident was that its optical identification with a spectroscopic binary HDE226868 has led to a presumption that it is a black hole. This possibility has induced continuous interests in the physical character of this source in conjunction with the nature of the black hole. The purpose of this paper is to summarize presently available pieces of knowledge on this source to help the design of future experimental and theoretical works, while the complexity of the source characteristics still has prevented us to construct a clear, coherent picture of this source in spite of the fact that numerous observational facts have been accumulated.