Abstract
We have studied the interaction of an X-ray flux with the gas in the stream that flows from the primary star to the secondary object in the system Cyg X-1 (HDE 226868). From the analysis of thermal and ionization equilibrium, it turned out that according to the X-ray characteristics, the gas might present instability conditions that induce phase transitions. The possibility of using this mechanism to explain the observations of the He x 4686 emission line was considered.
