X-ray study on stellar/intermediate mass black holes

Based on early Suzaku observations, properties of stellar-mass black-hole binaries (BHBs) and ultra-luminous compact X-ray sources (ULXs) are discussed. A comparison between GRO 1655-40 and Cygnus X-1, both observed in the Low/Hard state, gives new constraints on the characteristics of hot Compton cloud that are considered to be responsible for the formation of the Low/Hard state continua. Hard X-rays, with energies above 10 keV, were detected for the first time from a ULX, namely M82 X-1. Including this case, ULXs showing power-law like spectra are considered to host Comptonized accretion disks, in analogy to BHBs in the Very High state. In contrast, ULXs exhibiting multicolor-disk type spectra are interpreted to harbor 'slim' disks. This sort of analogy between ULXs and BHBs suggests that black holes in ULXs are significantly more massive than those in BHBs. (author)