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In this Section we examine evolution of the number of binary X-ray sources in an arbitrary galaxy. The first calculation of such an evolution was performed by Tatarintzeva et al. (1989).
We have assumed that an X-ray source appears in a binary in the following basic cases:
We also estimated the luminosity of the X-ray radiation from an isolated NS due to accretion from the interstellar medium. However, the problem with its X-ray radiation spectrum remains unsolved because no objects of this type have been observed so far. No intrinsic soft X-ray emission from pulsars (similar to that observed by ROSAT) was considered. But we take into account the absorption of radiation in the stellar wind from a normal companion and in the surrounding matter.
The total X-ray luminosity from different types of objects is traced separately at any instant. First, we calculated the evolution of the X-ray luminosity assuming a delta-function (instantaneous) shape for the star formation rate. Then, treating as a Green function, evolution of the X-ray luminosity from a galaxy with an arbitrary star formation rate can be represented as
The total hard X-ray luminosity is predominantly due to the binaries consisting of an accreting NS with a giant or supergiant companion which does not fill its Roche lobe (``A+II'' objects). The total number of such objects is in a good agreement with the observational data (see Table 7).
The X-ray luminosity evolution after an instantaneous star formation burst for t >2 yr can be rather well fitted by a power law:
(here N is the total number of stars in the galaxy). This fairly strong dependence on time in principle may be observed in remote elliptical galaxies.