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 derived 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 real data of the black hole binaries Cygnus X-1 and GX 339-4 and compare the results with their Fourier coherence spectra.