X-Ray Variability Coherence in the Time Domain

A technique for calculation of variability coherence at different timescales performed directly in the time domain is introduced. Simulations are made to compare the coherence spectrum derived by the time domain technique with the coherence function by Fourier analysis. The results indicate that in comparison with the Fourier analysis the time domain technique is more powerful for revealing signal coherence in noisy data. We apply the time domain technique to the real data of the black hole binaries Cygnus X-1 and GX 339-4 and compare the results with their Fourier coherence spectra.