determining of the mean values of ${\sigma^2} \over T$ was performed by averaging over a large number of points in the spectrum, the error of the mean itself can be ignored, while the significance of the identification of a periodicity is determined by the ratio ${\sigma_p/\sigma_{\sigma}}$.

Results of x-ray transient pulsars observations in the Prognoz 9 and GRIF experiments
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The Transient Pulsars under Consideration and the Criterion of the Detection of Significant Flux.
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Using the method described above, all the data obtained in the Prognoz 9 and GRIF “Mir” experiments were analyzed. To select the significant periodicities the condition that the amplitude of the peak corresponding to the main period should exceed the mean noise value ${\sigma^2}_{noise} \over T$ by more than $k\sigma_{\sigma}$ was established. For each observation the real amplitude distribution of peaks in the search interval was analyzed (amplitude of points was measured in $\sigma_{\sigma}$ units). Thus the level of significance was chosen according to the characteristics of peak amplitude distribution individually for each frequency spectrum. The reason of such approach is that the nature of a significant part of background on frequency spectra is not stochastic. It is connected with real processes in other frequency 