Reflection and reprocessing of X-ray source radiation by the atmosphere of the normal star in a binary system.

Summary. In a close binary system, up to 30% of the X-ray source radiation reaching the surface of the normal star is reflected. The remaining 70% is absorbed and is subsequently re-radiated as optical and ultra-violet radiation. In this paper, we consider the case of a semi-infinite plane atmosphere in a stream of hard X-rays, and we solve numerically the X-ray transfer equation. Photoabsorption and Compton scattering are taken into account. The albedo of the reflected X-rays is found; we have also obtained their spectral, polarizational, spatial and temporal (for a pulsation primary flux) characteristics. The results obtained are discussed with specific reference to Her X 1, Cyg X3 and 2U 1700-37. We note that there may well exist X-ray sources which are weak , but bright at -:20keV; in the case of Her Xi, there is an interval of 24 days (out of a total of 36) when the Earth is not in the beam of X-ray pulsar: the system is then visible only by virtue of the X-ray flux reflected by the surface of HZ Her, and it may then be such a source. We note also that the transformation of soft X-rays in the atmosphere of HZ Her cannot account for its optical variability which, however, can be easily explained by the reprocessing hard X-rays. Two sources of the optical fluctuations of HZ Her = Her Xi are also discussed. Key words: X-ray binary systems - radiative transfer - X-ray albedo.