= \sigma^{2}_{pp} + \sigma^{2}_{noise},$$ where $\sigma^{2}_{pp}$ and $\sigma^{2}_{noise}$ characterize the amplitudes of the corresponding components. In general, the dependence $\sigma^{2}(T)$ (periodogram) can be represented as the superposition of a noise continuum (ideally, a smooth function of $T$) and discrete peaks of the existing periodicities that correspond to the main period and its multiplies. The continuum in the periodogram is determined by a superposition of different (noise) components. One of such noise factors, shot noise ($\sigma^{2}_{shot}$), is related to the finite number of photons with a Poisson distribution recorded per bin. This component in the periodogram can be calculated from the mean count rates in the channels; it is linear for periodograms with a constant bin length, $$\sigma^{2}_{shot}(T) = \bar{N} \cdot {1\over\Delta} \cdot {T\over T_{max}},$$ where $\bar{N}$ is the mean count rate, $\Delta$ is the bin length. Clearly, this component is always present in the noise, and the observed continuum can not be lower. The excess over the “shot” component in the total noise continuum can be explained by non-periodic variations in the fluxes from the observed X-ray sources (Galactic noise) and by variations in the instrumental background. For a wide range of periods, the results of data processing by the 