Radiative Processes, Spectral States and Variability of Black-Hole Binaries

We review radiative processes responsible for X-ray emission in hard (low)and soft (high) spectral states of black-hole binaries. The main process in thehard state appears to be scattering of blackbody photons from a cold disk bythermal electrons in a hot inner flow, and any contribution from nonthermalsynchrotron emission is at most small. In the soft states, blackbody diskemission dominates energetically, and its high-energy tail is due to scatteringby hybrid, thermal/nonthermal electrons, probably in active regions above thedisk surface. State transitions appear to correspond to a variable inner radiusof the cold disk driven by changes of the accretion rate. The existence of twoaccretion solutions, hot and cold, in a range of the accretion rate leads tohysteresis in low-mass X-ray binaries.Comment: Proceedings of "Stellar-mass, intermediate-mass, and supermassive black holes", Kyoto, 10.200