for the Galactic Center. Figure 2 shows a circumference with a radius of $30^o$ whose center coincides with the Galactic Center. There are sources within the region in the sky bounded by this circumference during the observations of which the PX-2 effective area accounted for no less than $50\%$ of its geometric area. As one can see from the figure, the total observing time of the Galactic Centre with $\geq 50\%$ efficiency was $\sim 200$ h. The exposure times of other Galactic sources (for example, 4U1700-37) are similar.

The data processing technique
-----------------------------

To reveal the periodic processes the time sets of X-ray instrument outputs were analyzed. In the case of the Prognoz 9 experiment such primary outputs were the mean count rates for 10 s. In the case of “Mir” GRIF experiment primary outputs were the mean count rates for 5 s. These time sets were subject to random and regular variations, which produce the background for periodicities. These include rises in the X-ray flux due to solar flares and cosmic gamma-ray bursts, changes of the background count rate in the X-ray channels caused by variations of charged particle fluxes that bombarded the spacecraft, and some others. The counts 